py: Simplify and improve mp_get_index.

It has (again) a fast path for ints, and a simplified "slow" path for
everything else.

Also simplify the way str indexing is done (now matches tuple and list).
py: Fix up object equality test.

It regressed a bit after implementing float/complex equality.  Now it
should be improved, and support more equality tests.
py: Convert some macros to inline functions (in obj.h).

Also convert mp_obj_is_integer to an inline function.

Overall this decreased code size (at least on 32-bit x86 machine).