go regexp + RE2 DFA port

import "matloob.io/regexp"

See golang.org/cl/12081

• The regexp tests pass. Though there may still be uncaught bugs. Let me know if you find any of them! No guarantees!
• regexp/internal/dfa tests are currently failing. I need to fix some thingsn there.
• I've got a small change to the DFA that uses package unsafe and makes matches 2x faster. I'll try to get it up soon.
• A bunch of cleanup needs to be done all over this package.