Final touches to the API

- Removed reference to *Context in Number's since this scheme couldn't
  be applied to other number types like Quads (too much overhead)
- NumberPool is now fully exported
- Added *Context to NumberPool as an anonymous field. This way we can use
a single variable to keep track of free-list AND Context.
- Rewrote Example2 in its own file to make it clearer.
- Renamed package to "dec", keeping it nice and short.