Add support for exposing decorated services

[adamjones2]

Resolves #184.

I wrapped the service key in an opaque handle in case you want to change the implementation of how this works again in future (it also makes it easier to ferry around the static type information).

I did feel kind of bad about duplicating every single Decorate and TryDecorate method but I couldn't see any other way to emit the information needed short of a hack like dropping it in some passive ambient state in a static class or something. I preferred the out parameter route.