rust.eclass: Introduce new eclass for slotted Rust

The rust eclass acts similarly to the llvm eclass.

It works with optional `RUST_{MAX,MIN}_SLOT` variables to
enable ebuilds to trivially dependencies on appropriate
Rust SLOTs.

A `RUST_NEEDS_LLVM` variable can be set to have the eclass read
`LLVM_COMPAT` and generate an llvm-r1-USE-gated dependency string.

`RUST_USEDEP`, if set will be included in the dependency string
and checks against installed packages.

In either case these are stored in `RUST_DEPEND` for consumpion
in the ebuild, and added to BDEPEND unless unless `RUST_OPTIONAL`
is set. `RUST_DEPEND` will only ever include in-tree slots; there
is no need to set `RUST_MIN_VER` if all in-tree versions are suitable.
If `RUST_MIN_VER` is set to a value older than the oldest in-tree slot
it is treated as if it is set to the lowest available.

The default `rust_pkg_setup` will prefix the selected slot to
`PATH` and export `RUSTC` and `CARGO` variables pointing to that
slot for ease-of-use.

This should prevent issues like:

Bug: https://bugs.gentoo.org/907492
Bug: https://bugs.gentoo.org/942444
Signed-off-by: Matt Jolly <kangie@gentoo.org>

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