Decorator / attribute functions framework

This is generic part of a feature we have been discussing few days ago. Probably a bit overly generic but that D metaprogramming is like drugs - very hard to stop once addiction is up and running :(

Unittest examples should provide a clear picture how this is going to be used for enhancing the REST module (this is work-in-progress currently).

Slightly off-topic; do you think something like that may fit into Phobos after some field testing or it feels too weird for stdlib?