Add special implementation for small integers?

[skirpichev]

See flint, cpython and faster-python/ideas.

E.g.
https://github.com/flintlib/flint/blob/3d3f1f08b1f40c01d8e7de54741357ee6a704700/src/flint.h.in#L454-L455
(this assume that malloc() always allocates memory blocks on a or byte boundary)
and
https://github.com/flintlib/flint/blob/3d3f1f08b1f40c01d8e7de54741357ee6a704700/src/fmpz/mul.c#L149-L157