This patch introduces MP_PYTHON_PRINTER for general use.

Builtin functions with a fixed number of arguments (0, 1, 2 or 3) are
quite common.  Before this patch the wrapper for such a function cost
3 machine words.  After this patch it only takes 2, which can reduce the
code size by quite a bit (and pays off even more, the more functions are
added).  It also makes function dispatch slightly more efficient in CPU
usage, and furthermore reduces stack usage for these cases.  On x86 and
Thumb archs the dispatch functions are now tail-call optimised by the
compiler.

The bare-arm port has its code size increase by 76 bytes, but stmhal drops
by 904 bytes.  Stack usage by these builtin functions is decreased by 48
bytes on Thumb2 archs.

In order to have more fine-grained control over how builtin functions are
constructed, the MP_DECLARE_CONST_FUN_OBJ macros are made more specific,
with suffix of _0, _1, _2, _3, _VAR, _VAR_BETEEN or _KW.  These names now
match the MP_DEFINE_CONST_FUN_OBJ macros.

As defined in py/py.mk.

Now frozen modules generation handled fully by py.mk and available for reuse
by any port.

Concurrent WebREPL connections were never supported, now actually check
for this.

Running Python code on a hard interrupt is incompatible with having a GIL,
because most of the time the GIL will be held by the user thread when the
interrupt arrives.  Hard interrupts mean that we should process them right
away and hence can't wait until the GIL is released.

The problem with the current code is that a hard interrupt will try to
exit/enter the GIL while it is still held by the user thread, hence leading
to a deadlock.

This patch works around such a problem by just making GIL exit/enter a
no-op when in an interrupt context, or when interrupts are disabled.

See issue #2406.

It's probably not strictly needed so far, but serves as an example of
MP_FASTCODE use and may be helpful in the future.

It can be used in the following manner:

void MP_FASTCODE(foo)(int arg) { ... }

SDK 2.0.0 goes into boot loop if a firmware is programmed over erased flash,
causing problems with user experience. This change implements behavior
similar to older SDKs': if clean flash is detected, default system
parameters are used.

As part of this patch the MICROPY_HW_LED_OTYPE setting is removed because
it is now unused (all boards anyway had this as OUTPUT_PP).

Adds 1072 bytes to the code size.

To conform with Hardware API.

In particular remove the "*" because not all ports support keyword
arguments.

Reduces code size by 632 bytes.

It now works.

Also adds "machine" to the list of modules that the parser can search for
constants.

Setting the pyexec_system_exit variable to PYEXEC_FORCED_EXT allows
SystemExit exceptions to terminate the pyexec functions.

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.

The failed key is available as exc.args[0], as per CPython.

Iterables don't respond to __len__, so call __len__ on the original
argument.

Accepts only value of True.