stdscript: Add v0 p2pkh-schnorr-secp256k1 benchmark.
stdscript: Add v0 p2pkh-schnorr-secp256k1 benchmark.

BenchmarkIsPubKeyHashSchnorrSecp256k1Script
-------------------------------------------
v0_complex_non_standard        187337484   6.345 ns/op   0 allocs/op
v0_p2pkh-schnorr-secp256k1     120387538   10.07 ns/op   0 allocs/op
v0_p2pkh-schnorr-secp256k1_2   100000000   10.32 ns/op   0 allocs/op

This is part of a series of commits to fully implement the stdscript
package.

decred/dcrd,decred/dcrd