Use specialized formulas for doubling

The new function is about 20% faster, and signature
verification is now about 7% faster.
Merge pull request #15 from gnprice/doubling

Use specialized formulas for doubling
Space out *'s, change ranges to xranges'
Speed up by fast unpacking

Replace manual bit unpacking of integers with
C-speed unpacking using the struct module.
Add a simple profiling script