This commit adds the math.factorial function in two variants:
- squared difference, which is faster than the naive version, relatively
  compact, and non-recursive;
- a mildly optimised recursive version, faster than the above one.

There are some more optimisations that could be done, but they tend to take
more code, and more storage space.  The recursive version seems like a
sensible compromise.

The new function is disabled by default, and uses the non-optimised version
by default if it is enabled.  The options are MICROPY_PY_MATH_FACTORIAL
and MICROPY_OPT_MATH_FACTORIAL.

So that a pointer to it can be passed as a pointer to math_generic_1.  This
patch also makes the function work for single and double precision floating
point.

This patch changes how most of the plain math functions are implemented:
there are now two generic math wrapper functions that take a pointer to a
math function (like sin, cos) and perform the necessary conversion to and
from MicroPython types.  This helps to reduce code size.  The generic
functions can also check for math domain errors in a generic way, by
testing if the result is NaN or infinity combined with finite inputs.

The result is that, with this patch, all math functions now have full
domain error checking (even gamma and lgamma) and code size has decreased
for most ports.  Code size changes in bytes for those with the math module
are:

   unix x64:  -432
unix nanbox:  -792
      stm32:   -88
    esp8266:   +12

Tests are also added to check domain errors are handled correctly.

This comment style is no longer used because the docs are written by hand,
not generated.

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

Previous to this patch, if the result of the trunc/ceil/floor functions
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.

All float constants in the core should use this macro to prevent
unnecessary creation of double-precision floats, which makes code less
efficient.

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

In some compliation enviroments (e.g. mbed online compiler) with
strict standards compliance, <math.h> does not define constants such
as M_PI.  Provide fallback definitions of M_E and M_PI where needed.

With this patch the n_args parameter is changed type from mp_uint_t to
size_t.

These functions will raise 'ValueError: math domain error' on invalid
input.

Will be included only when MICROPY_PY_MATH_SPECIAL_FUNCTIONS is enabled.

Also covers cmath module (but only log10 is there at the moment).

It's safer to define it where it's used, defining it for all source files
may lead to hard to diagnose conflicts in corner cases.

The implementation of these functions is very large (order 4k) and they
are rarely used, so we don't enable them by default.

They are however enabled in stmhal and unix, since we have the room.

Addresses issue #1022.

This is just a clean-up of the code.  Generated code is exactly the
same.

See discussion in issue #50.

It defines types used by all other headers.

Fixes #691.

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.

Now of the form MICROPY_PY_*.  See issue #35.

Fix some unused variables, and silence a clang warning about initialization override in vmentrytable.h

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/.

Not all functions implemented.  Not enabled on pyboard.

Towards addressing issue #424.

Had a small increase to ROM usage (order 60 bytes).

Pretty much everyone needs to include map.h, since it's such an integral
part of the Micro Python object implementation.  Thus, the definitions
are now in obj.h instead.  map.h is removed.

Comes with some refactoring of code and renaming of files.  All modules
are now named mod*.[ch].