Update lib/Beam/Emitter.pm

Faster and more semantically correct aliasing of on/un.

While `goto &sub` is fun, it adds one more layer of indirection. Unless you need to choose what method to delegate to at runtime, a direct (typeglob) alias should be faster.
Merge pull request #1 from jberger/patch-1

Faster and more semantically correct aliasing of on/un