Add support for lazily initialized statics

[SimonSapin]

@Kimundi published a macro for "lazily initialized statics":

https://gist.github.com/Kimundi/8782487
http://www.reddit.com/r/rust/comments/1wvxcn/lazily_initialized_statics/

Besides the "lazy" aspect, it is useful to have statics (globals) whose value is not a "constant expression". The latest revision only compiles with Share types, which I believe makes it memory-safe.

I’d like this to be added to libstd or libsync. (It depends on sync::one::Once.)

[SimonSapin]

This is now a stand-alone library that is very easy to use with Cargo: https://github.com/Kimundi/lazy-static.rs