Move INT_FMT, etc. declaration into global mpconfig.h .

This in particular makes it available for stm port.
Merge pull request #56 from pfalcon/int_fmt-global

Move INT_FMT, etc. declaration into global mpconfig.h .
Merge pull request #52 from chipaca/master

Tweaks to list.pop.
Add basic implementation of slice object.

So far, only start and stop integer indexes are supported. Step is
not supported, as well as objects of arbitrary types.
Implement BUILD_SLICE opcode (2-arg version).
str: Initial implementation of string slicing.

Only step=1 and non-negative indexes are supported so far.
Enable slice support in config.
slice: Implement special handling of omitted start/stop indexes.
str: Handle non-positive slice indexes.
Add test for byte string slicing.
slice: Allow building with MICROPY_ENABLE_SLICE=0.
str: Throw TypeError for invalid index type and clean up comments.
Merge pull request #58 from chipaca/list_clear

Added list.clear. Fixes issue #53.
Merge pull request #59 from pfalcon/slice

Implement basic slice object and string slicing
Fix int -> machine_int_t; add print to slice test.
Merge remote-tracking branch 'upstream/master' into list_insert