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

Found this bug by running unix/ tests with DEBUG=1 enabled when
compiling.

Saves ROM (16 on stmhal, 240 on 64-bit unix) and should be quicker since
there is 1 less branch.

Because (for Thumb) a function pointer has the LSB set, pointers to
dynamic functions in RAM (eg native, viper or asm functions) were not
being traced by the GC.  This patch is a comprehensive fix for this.

Addresses issue #820.

Viper functions can now be annotated with the type of their arguments
and return value.  Eg:

@micropython.viper
def f(x:int) -> int:
    return x + 1

reversed function now implemented, and works for tuple, list, str, bytes
and user objects with __len__ and __getitem__.

Renamed mp_builtin_len to mp_obj_len to make it publically available (eg
for reversed).

This happens for example for zero-size arrays. As .get_buffer() method now
has explicit return value, it's enough to distinguish success vs failure
of getting buffer.

Addresses issue #724.

This allows to create str's with a smaller length than initially asked
for.

The user code should call micropython.alloc_emergency_exception_buf(size)
where size is the size of the buffer used to print the argument
passed to the exception.

With the test code from #732, and a call to
micropython.alloc_emergenncy_exception_buf(100) the following error is
now printed:
```python
>>> import heartbeat_irq
Uncaught exception in Timer(4) interrupt handler
Traceback (most recent call last):
  File "0://heartbeat_irq.py", line 14, in heartbeat_cb
NameError: name 'led' is not defined
```

See discussion in issue #50.

This will allow roughly the same behavior as Python3 for non-ASCII strings,
for example, print("<phrase in non-Latin script>".split()) will print list
of words, not weird hex dump (like Python2 behaves). (Of course, that it
will print list of words, if there're "words" in that phrase at all, separated
by ASCII-compatible whitespace; that surely won't apply to every human
language in existence).

Should finish addressing issue #524.

Also, make sure that args to "*" format specifiers are bounds-checked
properly and don't lead for segfaults in case of mismatch.

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.

This removes need for some casts (at least, more than it adds need
for new casts!).

This means that complete slice operations are supported for lists (but not
for bytearray's and array.array's).

Older int-only encoding is not expressive enough to support arbitrary slice
assignment operations.

Addresses issue #598.

See issue #608 for justification.

Done in generalized manner, allowing any stream class to be specified as
working with bytes.

Likely there are other functions that should be renamed, but this is a
start.

Two things are handled here: allow to compare native subtypes of tuple,
e.g. namedtuple (TODO: should compare type too, currently compared
duck-typedly by content). Secondly, allow user sunclasses of tuples
(and its subtypes) be compared either. "Magic" I did previously in
objtype.c covers only one argument (lhs is many), so we're in trouble
when lhs is native type - there's no other option besides handling
rhs in special manner. Fortunately, this patch outlines approach with
fast path for native types.

Slice value to assign can be only a list so far too.

Tired of patching CPython stdlib for it.

Blanket wide to all .c and .h files.  Some files originating from ST are
difficult to deal with (license wise) so it was left out of those.

Also merged modpyb.h, modos.h, modstm.h and modtime.h in stmhal/.