Move regexp.MustCompile outside of function

This ensures that the regex is only compiled once. Rather than on every
lookup. Significantly faster:
```
BenchmarkLookup  1000000              1490 ns/op
BenchmarkLookupBogus     1000000              2343 ns/op
BenchmarkLookupMalicious          100000             28128 ns/op
ok      github.com/alphagov/router/triemux      7.164s
```