User:GYakovlev/Rust bump

Attempt to document dev-lang/rust bump procedure (WIP):

high-level action item:

1. bump dev-lang/rust-bin
2. bump dev-lang/rust
3. bump virtual/rust
4. bump sys-devel/rust-std

Detailed steps for source ebuild:

1. examine https://github.com/rust-lang/rust/blob/master/config.example.toml for any recent changes since last release, update src_configure() accordingly.
2. update/rebase/cleanup patches as needed.
3. build test with USE="clippy doc nightly rls rustfmt system-bootstrap verify-sig miri wasm"
4. compile couple of high-profile packages: cbindgen ripgrep spidermonkey firefox
5. now build with USE=system-llvm
6. compile high-profile packages again
7. compile with and without USE=system-bootstrap.
8. test is current version can bootstrap itself, not only version-1. It's a common failure in bootstrap code.
9. compile with FEATURES=test ERUST_RUN_EXTRA_TESTS=yes and check testsuite output, there will be some errors, look for codegen and critical ones. you will need ALL LLVM_TARGETS set for testsuite.
10. repeat generic build, package compilation and testsuite on at least 1 alternative arch, arm64/ppc64 are good targets.
11. compile on musl, amd64 or arm64, build ripgrep.
12. update pre_build_checks() with new sizes if needed.
13. check eselect-rust is working
14. bump the rest (easy) and push