primitives: Add hash to uint256 benchmark.
primitives: Add hash to uint256 benchmark.

The following is a comparison between the existing implementation in the
blockchain/standalone module with big integers (old) and the new
implementation (new) averaging 10 runs each:

name            old time/op     new time/op     delta
--------------------------------------------------------------------------
HashToUint256   127ns ± 3%      3ns ± 1%        -97.68%  (p=0.000 n=10+10)

name            old allocs/op   new allocs/op   delta
---------------------------------------------------------------------------
HashToUint256   2.00 ± 0%       0.00            -100.00%  (p=0.000 n=10+10)

decred/dcrd,decred/dcrd