Unifying workspaces for tools and others

[ishitatsuyuki]

Compiling cargo is known to be slow as hell, and it would be better if we build less.

I'm proposing here to make a cargo workspace for all the stage2 tools. This way,

• If well configured, Cargo will only be built once (shared with RLS build artifacts)
• More parallelism since there's more crates to build at once and less likely to block on single big crate

Also, I'm not sure if it's possible, but grouping std, rustc and minor tools can also improve a bit.

[ishitatsuyuki]

@alexcrichton I would appreciate some discussion here. By the way, the dream is blocked by rust-lang/cargo#3976.

BTW, I'm currently working on a local PoC.

[alexcrichton]

I don't really understand what the issue here is about, but faster builds are faster builds!