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.

The LHS passed to mp_obj_int_binary_op() will always be an integer, either
a small int or a big int, so the test for this type doesn't need to include
an "other, unsupported type" case.

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.

The unary-op/binary-op enums are already defined, and there are no
arithmetic tricks used with these types, so it makes sense to use the
correct enum type for arguments that take these values.  It also reduces
code size quite a bit for nan-boxing builds.

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

Before this patch raising a big-int to a negative power would just return
0.  Now it returns a floating-point number with the correct value.

The standard preprocessor definition to differentiate debug and non-debug
builds is NDEBUG, not DEBUG, so don't rely on the latter:
- just delete the use of it in objint_longlong.c as it has been stale code
  for years anyway (since commit [c4029e50]): SUFFIX isn't used anywhere.
- replace DEBUG with MICROPY_DEBUG_NLR in nlr.h: it is rarely used anymore
  so can be off by default

This reduces code duplication and allows to make mp_classify_fp_as_int
static, which reduces code size.

This makes int.from_bytes() work for MICROPY_LONGINT_IMPL_LONGLONG.

To be implemented later.

And also simplify it to remove the check for small int.  This can be done
because this function is only ever called if the argument is not a small
int.

Using the original WORD_MSBIT_HIGH-logic resulted in errors when the
object model is not REPR_A or REPR_C.

This function returns the sign (-1, 0 or 1) of the integer object.

Hashing is now done using mp_unary_op function with MP_UNARY_OP_HASH as
the operator argument.  Hashing for int, str and bytes still go via
fast-path in mp_unary_op since they are the most common objects which
need to be hashed.

This lead to quite a bit of code cleanup, and should be more efficient
if anything.  It saves 176 bytes code space on Thumb2, and 360 bytes on
x86.

The only loss is that the error message "unhashable type" is now the
more generic "unsupported type for __hash__".

This gets int.to_bytes working for bignum, and also struct.pack with 'q'
and 'Q' args on 32-bit machines.

Addresses issue #1155.

Addresses issue #1022.

This fixes conversion when float type has more mantissa bits than small int,
and float value has small exponent. This is for example the case of 32-bit
platform using doubles, and converting value of time.time(). Conversion of
floats with larg exponnet is still not handled correctly.

mp_obj_int_get_truncated is used as a "fast path" int accessor that
doesn't check for overflow and returns the int truncated to the machine
word size, ie mp_int_t.

Use mp_obj_int_get_truncated to fix struct.pack when packing maximum word
sized values.

Addresses issues #779 and #998.

Addresses issue #848.

Type representing signed size doesn't have to be int, so use special value
which defaults to SSIZE_MAX, but as it's not defined by C standard (but rather
by POSIX), allow ports to set it.

Addressing issue #50, still some way to go yet.

Addresses issue #765.

Implementing it as a static constant is a bit peculiar and require cooperation
from long int implementation.

See discussion in issue #50.

This renames:
MICROPY_PY_FROZENSET -> MICROPY_PY_BUILTINS_FROZENSET
MICROPY_PY_PROPERTY -> MICROPY_PY_BUILTINS_PROPERTY
MICROPY_PY_SLICE -> MICROPY_PY_BUILTINS_SLICE
MICROPY_ENABLE_FLOAT -> MICROPY_PY_BUILTINS_FLOAT

See issue #35 for discussion.

Also unifies use of SMALL_INT_FITS macro across parser and runtime.

Addresses issue #627.

See issue #608 for justification.