GROOVY-6360: move @Pure to separate subproject.

[paulk-asert]

[paulk-asert]

Draft, still needs some work.

[paulk-asert]

I'll close this. The intention of moving this to a separate jar was so other folks could start using the annotation without depending on (all of) Groovy. I am not sure the world is a better place with yet another choice of a general purpose annotation library. We'll support Groovy's annotation if you take the Groovy jar and we'll work out which other libraries to support as a separate issue.