Parallel rustc

[nikomatsakis]

As @Zoxc's work to [parallelize rustc] starts to land, we need to start documenting it!

Some example topics:

• Copy some of the text from the internals thread as to the overall strategy
• What the heck is Lrc? When should I be using it?
• What do I use when I want a ref cell?

[mark-i-m]

Perhaps a good starting point might be https://github.com/nikomatsakis/rustc-parallelization/blob/master/interior-mutability-list.md and the discussion in https://internals.rust-lang.org/t/parallelizing-rustc-using-rayon/6606/24

[SparrowLii]

#1432 adds the use of thread-safe data structures and parallel iterators, and corrects some statements in other parts. In addition, the reason for using the fork version of rayon needs to be added to the documentation.