unix: Allocate more heap memory by default on 64 bit machines.

Pointers are 2x bigger on 64 bit machines, so we should allocate twice
the memory to have a comparable heap size.
py: Remove mp_obj_less (use mp_binary_op(MP_BINARY_OP_LESS..) instead).
py: Allow dict constructor to take keyword arguments.
py: Make False and True act like 0 and 1 for integer arithmetic.