Vendor dependencies

## Progress

- [ ] Vendor normal dependencies (#2883)
- [ ] Vendor std dependencies for `-Zbuild-std[=core]`
- [ ] Update cargo-semver-checks to check against in-tree version of zerocopy (using `--baseline-rev`) so we don't have to bypass vendoring in order to run `cargo semver-checks`.

## Details

This will solve a number of problems:
- Simplify the logic which ensures that we never depend on crate versions which are incompatible with our MSRV
- In local development, avoid a performance bottleneck from updating the crates.io index (on the MSRV toolchain, this is a very slow process if the index is far out of date)
- In CI, avoid having to download dependencies
- Resolve #1595