add rename and Builder.rename

[justinwoo]

Fixes #17

[MonoidMusician]

would it be more efficient to use the builder method to implement the regular method? I think that would avoid one copying of the record, something like rename prev next = Builder.build (Builder.rename prev next) ...

[justinwoo]

Somehow I still like the pure versions a lot more than the builder versions though

[paf31]

Thanks!