Rust installer v2

This upgrades to a version of [rust-installer](https://github.com/rust-lang/rust-installer) that supports multiple 'components', so that the output can be combined into a [single installer](https://github.com/rust-lang/rust-packaging) that contains both Rust and Cargo (#16456).

At the same time it also packages documentation into a component called 'rust-docs', which will again be merged into the combined installer (#14916).

It creates yet *another* component, 'rust-mingw', to carry the optional parts of mingw necessary to link with Rust on windows, and makes distcheck build all these installer components for upload by the buildbots. It also moves the 'third-party' docs from the 'bin/' folder to 'share/doc/rust' where the rest of the docs live, temporarily removing them from the .exe installer.

This needs to be merged at roughly the same time as the [corresponding Cargo PR](https://github.com/rust-lang/cargo/pull/1102).

Next steps:

* Modify rust-packaging to produce combined .exe and .pkg installers.
* Set up automation for building rust-packaging every time a rust build is completed.
* Rename the 'rust' installer artifacts produced here to 'rustc' and the the 'rust-combined' produced by rust-packaging to just 'rust'.
* Remove the .exe and .pkg packaging from the main repo.

r? @alexcrichton p=1 please. There's a lot more to do here.
Above this used to be `[ ! -d doc ] || ...` , was it intentional to lose the prefix?