Add remaining CI improvements

[josephlr]

After #192, we still have a few things we should add to our CI. These include:

• Running the iOS binaries we build (ideally on both x86_64 and aarch64) CI: Run iOS targets and other improvements #316
• Build and run aarch64-apple-darwin binaries (aka "Apple Silicon") CI: Run tests on aarch64-apple-darwin and aarch64-apple-ios-sim. #398
• Add emscripten tests (Add Emscripten tests #196)
• Add no-std test for a custom hook (Add test for custom RNG handler #197)
• Cache toolchain downloads (see Caching tracking issue actions-rs/meta#21 and Advice on toolchain caching actions-rs/toolchain#54) Not useful with move to https://github.com/dtolnay/rust-toolchain
• Cache crates.io downloads/index (Add fixes for Caching and rustc-dep-of-std feature #198)

[josephlr]

With #316 the only think we are waiting on is AArch64 macOS runners. This is tracked in github/roadmap#528