showbc: Update for recent int varlen storage refactor.

TODO: De-duplicate DECODE_UINT, etc. definitions.
parse: Note that fact that parser's small ints are different than VM small int.

Specifically, VM's small ints are 31 bit, while parser's only 28. There's already
MP_OBJ_FITS_SMALL_INT(), so, for clarity, rename MP_FIT_SMALL_INT() to
MP_PARSE_FITS_SMALL_INT().