Add string formatting support for longlong and mpz.

Tested under unix with NONE, LONGLONG and MPZ implementations.
Added comma format support
'{:6,d}'.format(1234) now produces ' 1,234'

TODO: a function which converts float/double to mpz integer.

Right now '%d' % 1e20 produces '0' under micropython and '100000000000000000000' under python.