minor efficiency improvements through more use of bit shift operations

I have added further bit shift operations (where possible) to increase the efficiency of arithmetic operations. The code base is also used by the Ed25519 EdDSA implementation, and has passed the full test suite including the 1024 Ed25519 test vectors from the original Python implementation. Therefore, I'm quite certain that the changes should not break compatibility with anything.

https://github.com/meganz/ed25519js

Hope that helps.