Dont compile the regexp for every ParseHex call.

The regexp object is threadsafe for use in multiple goroutines
so it can be compiled during program init.

Added a benchmark to test performance.

Before:
BenchmarkParseHex-4	  500000	     41549 ns/op	   21190 B/op	     115 allocs/op

After:
BenchmarkParseHex-4	 5000000	      4332 ns/op	     336 B/op	       6 allocs/op
Merge pull request #15 from Zariel/speed-up-parse-hex

Dont compile the regexp for every ParseHex call.