stdscript: Add v0 p2sh benchmark.
stdscript: Add v0 p2sh benchmark.

BenchmarkIsScriptHashScript
---------------------------
v0_complex_non_standard   923756949   1.279 ns/op   0 allocs/op
v0_p2sh                   746887346   1.502 ns/op   0 allocs/op

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

decred/dcrd,decred/dcrd