EC constants and bitShifts

Chained on #182.

This pull request is relatively straight forward, it extracts several `BigInteger` constants in `ec.js` and `ecdsa.js`.
It also uses a `shiftRight` in place of `divide` where the divisor was a multiple of 2, as it is almost 15x faster, and poses a significant performance penalty.

Mostly just some low hanging fruit.