py: Implement sys.maxsize, standard way to check platform "bitness".

Implementing it as a static constant is a bit peculiar and require cooperation
from long int implementation.
modsstruct: Foreign data interface module, roughly based on ctype ideas.

But much smaller and memory-efficient. Uses Python builtin data structures
(dict, tuple, int) to describe structure layout.

Currently, the only available docs are testcases.
modsstruct: sizeof(): Fix handling of recursive structs.
modsstruct: sizeof(): Operate on sstruct objects too (not just on layouts).
modsstruct: Replace some asserts with exceptions.
modsstruct: Implement sizeof() for PTR and ARRAY.
modsstruct: More docstrings for gendoc.py.
modsstruct: 64-bit cleanness.
modsstruct: Add VOID type value, for void pointers.

Actually implemented as an alias for UINT8 for now.
modsstruct: Allow to specify structure layout type (native/LE/BE).
modsstruct: Follow machine_int_t -> mp_int_t rename.