Reenable optimized build.
Shrink Key structure, implement __sizeof__ (2nd try)

This reduces Key from 48 bytes to 32 bytes, introduces maximum size
limit of 64kb, and sys.getsizeof() accurate calculation.

First commit, a006c0f422cada9a984161a81d6b0d1448a8bbc4, accidentally
replaced pymalloc calls with system allocator, causing 12% slowdown(!).

Closes #72