[gentoo-dev] Add rust eclass to support multi-target compilation

[gibix]

Hello,

I opened a PR here: https://github.com/gentoo/gentoo/pull/9388

Here a copy of the message:

# add support for rust multi-target

- add rust.eclass (with rust-utils.class) to support rust multi-target with
  multibuild
- modify cargo.class to support rust multi-target
- change dev-lang/rust slot system

This will allows projects like rustfmt, clippy, bindgen that need runtime
linking with the proper rust version to work correctly. Beyond this while
rust is getting older as project we will see more projects that will
require a specific rust version for compilation.

This PR replaces the need for rustup in gentoo for toolchain handling and
components.

requires:
- [features in
  cargo-ebuild](https://github.com/cardoe/cargo-ebuild/pull/14/commits/1aefd302f9430c0b628923da23e2a8d74b83d1ec)
- [binaries support into
  eselect-rust](https://github.com/jauhien/eselect-rust/pull/4)

see first discussion on
[gentoo-rust](https://github.com/gentoo/gentoo-rust/pull/362)