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 #59 from pfalcon/slice

Implement basic slice object and string slicing