regexp: speed up by about 30%.

The code used interfaces in a pretty, pedagogical way but not efficiently.
Remove unnecessary interface code for significant speedups.
Before:

	regexp.BenchmarkLiteral	 1000000	      2629 ns/op
	regexp.BenchmarkNotLiteral	  100000	     18131 ns/op
	regexp.BenchmarkMatchClass	  100000	     26647 ns/op
	regexp.BenchmarkMatchClass_InRange	  100000	     27092 ns/op
	regexp.BenchmarkReplaceAll	  100000	     27014 ns/op

After:

	regexp.BenchmarkLiteral	 1000000	      2077 ns/op
	regexp.BenchmarkNotLiteral	  100000	     13738 ns/op
	regexp.BenchmarkMatchClass	  100000	     20418 ns/op
	regexp.BenchmarkMatchClass_InRange	  100000	     20999 ns/op
	regexp.BenchmarkReplaceAll	  100000	     21825 ns/op

There's likely more to do without major surgery, but this is a simple, significant step.

R=rsc
CC=golang-dev
https://golang.org/cl/3572042