Upgrade rust-installer to v2
Install copyright information and package docs

This distributes docs in a separate package called rust-docs. The rust-packaging
project will combine it with Rust and Cargo into a single installer in a variety of formats.
mk: Make distcheck build binary tarballs on windows

These work, but aren't being built and uploaded because the bots run
'distcheck' not 'dist'.
mk: Package mingw components in unix installer on windows

This puts stdc++ and the unwinding dll into the main package
and creates a separate rust-mingw package for everything else.
Simplify some logic in dist.mk
rollup merge of #20317: brson/rust-installer-v2