refactored rust_toolchain into rust_exec_toolchain and rust_target_toolchain

[UebelAndre]

Required pull-requests:

• create_universe no longer assumes the name of rust_repository repos #805
• Organized examples #803
• Added rust_stdlib_filegroup rule, a helper for creating toolchains #802
• Added crate attribute to rustdoc rules (deprecated dep) #799
• The toolchain argument of collect_deps was unused. Removing it. #798
• Exposed supported platform triples lists #797
• Made clippy's output file more look less like a target. #795
• Added checksums for additional rust assets to internal list #794
• Promoted internal _tidy function to the utils module as dedent #793
• Added standalone targets for rust_toolchain components. #792
• Added util module for rust repository rules. #773

[UebelAndre]

Just so everyone's aware of where all the PRs came from. I've been working on refactoring the toolchains so it's easier to register target toolchains for cross-compilation. There's still some rough edges here but the gist of this is the previous rust_toolchain has been split into exec and target variants where exec contains the compiler and things and target contains all the linkable libraries for the target platform. This is largely inspired by #523 and a conversation that was had in slack. As the other PRs get closed out, I'll be making minor changes here and adding documentation.

[hlopko]

I'm concerned that the approach you're taking may not be the best possible. Could you first start a discussion about:

• what you're trying to achieve?
• why is it important?
• how do you want to achieve it?

Whenever there is a big "chunk" of work that requires so many changes we should get into a habit of discussing the general approach first, and only starting coding afterwards. It's very frustrating to the author to receive substantial feedback in the middle of the "project" that invalidates prior work. Communicating up front makes it easier for the reviewers to understand the context and motivation behind changes.

[UebelAndre]

Closing this based on #815 (comment)