ci: new CI - #644

this PR remakes CI and simplifies it a great deal

it is a replacement for the rust.yml workflow

it adds two workflows:

• checks.yml => fast checks "everything is okay" for PRs. runs on PRs, merge queue, and manually
• performance.yml => deep benchmarking, runs on v2 push and manually

it deliberately changes tests/borsh.rs::round_trip_zst to have less elements so the miri test doesn't hang and reorders a block in src/taggedlen.rs so that there is no warning emitted

note: this PR is blocked by #631, and that's why checks currently fail

checks

checks has seven jobs:

• practices: runs rustfmt, clippy, and documentation build
• fuzz: runs fuzzing
• miri: runs all miri
• stable: runs <=default testing
• nightly: runs all testing
• nostd: runs <=default testing
• msrv: runs <=default testing

in total, these jobs are more comprehensive than our current checks

it removes the old beta testing. argument: why have it when we have both stable and nightly?? what's the point exactly?? it's the same as saying we should test every rust version since the MSRV or every nightly commit

it also removes the old "abort all other checks if one fails" so you can discover all issues at the same time

estimated run time: 1-2 minutes, around 8-10x faster than our current checks, which is perfect for PRs

performance

three jobs:

• no default features
• default features
• all features

running benchmarks simultaneously

estimated run time: 3-4 minutes, substantially faster than our current benchmarking because it executes in parallel