CI: Run iOS targets and other improvements

[josephlr]

Fixes most of #194 (except being about to run on Apple Silicon).

This changes multiple things in the CI, so it might be easier to review commit by commit:

• Move from actions-rs/toolchain to dtolnay/rust-toolchain
  • The current action we are using is unmantained: Project unmaintained actions-rs/toolchain#216
  • This replacement is faster, smaller, and more cachable.
  • We can also completely remove the toolchain step when testing with
    cross, as cross handles configuration of its own toolchain.
• Make job names consistent. This makes it easier to tell on which targets we:
  • Build
  • Link
  • Run
• Run iOS Simulator Tests
  • This builds, links, and runs the tests on x86_64-apple-ios using
    dinghy.
  • It moves the aarch64 targets which only Build/Link to their own job.
• Update to actions/checkout@v3
• Update to Swatinem/rust-cache@v2
• Break up Tier 3 builds into a job for each build.
  • Makes things more parallel