Rationalized API

- Context does not depend on Number anymore.
- NewNumber() is back with Number (requires more typing to use, but
  that's where it belongs).
- Replaced integrated free-list management by helpers for sync.Pool
  integration.
- Added a lightweight Pool implementation that can be used instead of
  sync.Pool.
- Module separation is complete. Ready for a real split should it
  become possible.
- Added GCFLAGS support to the Makefile