These macros could in principle be (inline) functions so it makes sense to
have them lower case, to match the other C API functions.

The remaining macros that are upper case are:
- MP_OBJ_TO_PTR, MP_OBJ_FROM_PTR
- MP_OBJ_NEW_SMALL_INT, MP_OBJ_SMALL_INT_VALUE
- MP_OBJ_NEW_QSTR, MP_OBJ_QSTR_VALUE
- MP_OBJ_FUN_MAKE_SIG
- MP_DECLARE_CONST_xxx
- MP_DEFINE_CONST_xxx

These must remain macros because they are used when defining const data (at
least, MP_OBJ_NEW_SMALL_INT is so it makes sense to have
MP_OBJ_SMALL_INT_VALUE also a macro).

For those macros that have been made lower case, compatibility macros are
provided for the old names so that users do not need to change their code
immediately.

Configurable via MICROPY_PY_BUILTINS_NEXT2, disabled by default.

Instead of redirecting to str.__mod__(), use str.format() in this case.

As per CPython semantics.  This feature is controlled by
MICROPY_PY_BUILTINS_ROUND_INT which is disabled by default.

If MICROPY_PY_ALL_SPECIAL_METHODS is enabled then dir() will now delegate
to the special method __dir__ if the object it is listing has this method.

This gives dir() better behaviour when listing the attributes of a user
type that defines __getattr__: it will now not list those attributes for
which __getattr__ raises AttributeError (meaning the attribute is not
supported by the object).

It now allows __getattr__ in a user type to raise AttributeError when the
attribute does not exist.

The list of qstrs starts with MP_QSTR_NULL followed by MP_QSTR_, and these
should never appear in dir() or REPL tab completion, so skip them.

This patch improves the builtin dir() function by probing the target object
with all possible qstrs via mp_load_method_maybe.  This is very simple (in
terms of implementation), doesn't require recursion, and allows to list all
methods of user-defined classes (without duplicates) even if they have
multiple inheritance with a common parent.  The downside is that it can be
slow because it has to iterate through all the qstrs in the system, but
the "dir()" function is anyway mostly used for testing frameworks and user
introspection of types, so speed is not considered a priority.

In addition to providing a more complete implementation of dir(), this
patch is simpler than the previous implementation and saves some code
space:

   bare-arm:   -80
minimal x86:   -80
   unix x64:   -56
unix nanbox:   -48
      stm32:   -80
     cc3200:   -80
    esp8266:  -104
      esp32:   -64

This patch provides inline versions of the utf8 helper functions for the
case when unicode is disabled (MICROPY_PY_BUILTINS_STR_UNICODE set to 0).
This saves code size.

The unichar_charlen function is also renamed to utf8_charlen to match the
other utf8 helper functions, and the signature of this function is adjusted
for consistency (const char* -> const byte*, mp_uint_t -> size_t).

The array should be of type unsigned byte because that is the type of the
values being stored.  And changing to uint8_t helps to prevent warnings
from some static analysers.

This allows the function to raise an exception when unknown keyword args
are passed in.  This patch also reduces code size by (in bytes):

   bare-arm:   -24
minimal x86:   -76
   unix x64:   -56
unix nanbox:   -84
      stm32:   -40
    esp8266:   -68
     cc3200:   -48

Furthermore, this patch adds space (" ") to the set of ROM qstrs which
means it doesn't need to be put in RAM if it's ever used.

This patch simplifies the str creation API to favour the common case of
creating a str object that is not forced to be interned.  To force
interning of a new str the new mp_obj_new_str_via_qstr function is added,
and should only be used if warranted.

Apart from simplifying the mp_obj_new_str function (and making it have the
same signature as mp_obj_new_bytes), this patch also reduces code size by a
bit (-16 bytes for bare-arm and roughly -40 bytes on the bare-metal archs).

Header files that are considered internal to the py core and should not
normally be included directly are:
    py/nlr.h - internal nlr configuration and declarations
    py/bc0.h - contains bytecode macro definitions
    py/runtime0.h - contains basic runtime enums

Instead, the top-level header files to include are one of:
    py/obj.h - includes runtime0.h and defines everything to use the
        mp_obj_t type
    py/runtime.h - includes mpstate.h and hence nlr.h, obj.h, runtime0.h,
        and defines everything to use the general runtime support functions

Additional, specific headers (eg py/objlist.h) can be included if needed.

This allows user classes to implement __abs__ special method, and saves
code size (104 bytes for x86_64), even though during refactor, an issue
was fixed and few optimizations were made:

* abs() of minimum (negative) small int value is calculated properly.
* objint_longlong and objint_mpz avoid allocating new object is the
  argument is already non-negative.

"STATIC inline" can expand to "inline" if STATIC is defined to nothing, and
this case can lead to link errors.

There were several different spellings of MicroPython present in comments,
when there should be only one.

The implementation is taken from stmhal/input.c, with code added to handle
ctrl-C.  This built-in is controlled by MICROPY_PY_BUILTINS_INPUT and is
disabled by default.  It uses readline() to capture input but this can be
overridden by defining the mp_hal_readline macro.

Working on a build with PY_IO enabled (for PY_UJSON support) but PY_SYS_STDFILES disabled (no filesystem).  There are multiple references to mp_sys_stdout_obj that should only be enabled if both PY_IO and PY_SYS_STDFILES are enabled.

Saves 168 bytes on bare-arm.

Previous to this patch, if the result of the round function overflowed a
small int, or was inf or nan, then a garbage value was returned.  With
this patch the correct big-int is returned if necessary and exceptions are
raised for inf or nan.

The C nearbyint function has exactly the semantics that Python's round()
requires, whereas C's round() requires extra steps to handle rounding of
numbers half way between integers.  So using nearbyint reduces code size
and potentially eliminates any source of errors in the handling of half-way
numbers.

Also, bare-metal implementations of nearbyint can be more efficient than
round, so further code size is saved (and efficiency improved).

nearbyint is provided in the C99 standard so it should be available on all
supported platforms.

Allows to iterate over the following without allocating on the heap:
- tuple
- list
- string, bytes
- bytearray, array
- dict (not dict.keys, dict.values, dict.items)
- set, frozenset

Allows to call the following without heap memory:
- all, any, min, max, sum

TODO: still need to allocate stack memory in bytecode for iter_buf.

Updated modbuiltin.c to add conditional support for 3-arg calls to
pow() using MICROPY_PY_BUILTINS_POW3 config parameter. Added support in
objint_mpz.c for for optimised implementation.

This builtin is configured using MICROPY_PY_BUILTINS_HELP, and is disabled
by default.

Saves the following number of bytes of code space: 176 for bare-arm, 352
for minimal, 272 for unix x86-64, 140 for stmhal, 120 for esp8266.

One can instead lookup __name__ in the modules dict to get the value.