Remove unnecessary includes. Add includes that improve portability.

See pull request #343.

Also modify mp_get_index to accept:
1. Indices that are or evaluate to a boolean.
2. Slice indices.
Add tests for these two cases.

If operation will overflow, a multi-precision integer is created.

A couple of bugs in mpn_shl, and overflow bug in mpz_set_from_int.

MICROPY_ENABLE_FLOAT is automatically set in mpconfig.h if MICROPY_FLOAT_IMPL
is set to a non-zero value.

mp_module_obj_t can now be put in ROM.

Configuration of float type is now similar to longint: can now choose
none, float or double as the implementation.

math module has basic math functions.  For STM port, these are not yet
implemented (they are just stub functions).

* Issue #322

Addresses issue #327.

Checks for non-default args following default args, and errors out.
Addresses issue #328.

I.e. as replacement of MP_OBJ_IS_TYPE(), which takes into account subclassing.

Implement not, shl and shr in mpz library.  Add function to create mpzs
on the stack, used for memory efficiency when rhs is a small int.
Factor out code to parse base-prefix of number into a dedicated function.