secp256k1: Improve scalar mult readability.
secp256k1: Improve scalar mult readability.

This slightly reworks the scalar multiplication logic to improve its
readability by making use of zero values and a single mask instead of
shifting every individual byte of the positive and negative portions of
k1 and k2.

It also improves the comments to clarify the intention of the code which
might not otherwise be obvious given the bit twiddling nature of it.

The reduction in the number of shifts in the double and add loop is also
technically an optimization, but the calculation is by far dominated by
the ec operations, so saving a few nanoseconds on shifts is so negigible
that it's in the margin of error in benchmarks.

decred/dcrd,decred/dcrd