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.toml.example 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