modstruct: Use proper int accessor which checks input value type.
modstruct: Add 'O' typecode for passing mp_obj_t.

Useful as callback data, etc.
modsocket: Add setblocking() method.
gc: Recover simple gc_realloc implementation, make easier to switch between.
gc: "new" gc_realloc: Rewrite in plain C, fixing bunch of bugs.

There were typos, various rounding errors trying to do concurrent counting
in bytes vs blocks, complex conditional paths, superfluous variables, etc.,
etc., all leading to obscure segfaults.
objcell: Add disabled by default print method for debugging.
objclosure: Store reference to entire closed variables tuple.

Avoids pointer-to-field garbage collection issue. Fixes #510.