Freeze all the things

WIP for 2.1.0

http://ruby.about.com/od/Whats-New-in-Ruby-210/fl/Whats-new-in-Ruby-210-Strings.htm

So far, performance benchmark shows no difference either way (on 2.1.0-dev), but I assume that's because the benchmarks are not really targeting this kind of thing.

cc @arthurnn