V2 manifests

[brson]

This teaches rustup.sh to read V2 manifests, adds the --with-target flag, which is used during the initial install to pull down additional standard libraries, the --add-target flag, which adds a target to an existing installation, and the --list-available-targets flag, which lists the targets available to an already-installed rust.

This has only been unit tested on Linux, Windows and Mac, not tested live, so don't merge yet.

I'll be happy to squash if this history is too ugly.

r? @alexcrichton cc @edunham

[brson]

Untabifying made the diff big. Here's one that starts from after untabifying: rust-lang/rustup@3c96043...next

[alexcrichton]

I think that <<< allows specifying a variable as stdin to a command so the routines that are creating a temporary file, reading it in, and then removing it may be able to use that instead? Not entirely sure if that works out with while loops though.

[alexcrichton]

r=me, although as usual wasn't necessarily heavily scrutinized, I trust this plus the level of tests though, so I'm pretty confident it'll work out!

[KennethAdamMiller]

Supposing you clone and then checkout branch next, and then try to use muslrust for a static build for a run-anywhere capability with:

./rustup.sh --with-target musl --prefix=./muslrust
rustc --target=x86_64-unknown-linux-musl test.rs

you get: error: can't find crate for std

How do I compile a hello world with musl rust if these steps don't work?