perf: cache the Vec used to build state keys
perf: cache the Vec used to build state keys

Avoids frequent re-allocations while the instructions are pushed to it.

rust-lang/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang/regex