txscript: Check equality via secp256k1 methods.
txscript: Check equality via secp256k1 methods.

This modifies the signature cache Exists method to make use of the new
efficient equality testing methods provided by the secp256k1 types
versus indirectly checking their equality via their serializations.

The following benchmark shows a before and after comparison of a typical
signature cache exists check:

name             old time/op     new time/op     delta
-------------------------------------------------------------------------
SigCacheExists   400ns ± 0%      62ns ± 0%       -84.54%  (p=0.000 n=5+5)

name             old alloc/op    new alloc/op    delta
--------------------------------------------------------------------------
SigCacheExists   256B ± 0%       0B              -100.00%  (p=0.008 n=5+5)

name             old allocs/op   new allocs/op   delta
--------------------------------------------------------------------------
SigCacheExists   4.00 ± 0%       0.00            -100.00%  (p=0.008 n=5+5)

decred/dcrd,decred/dcrd