I think this is the final attempt of mine to do anything about the
binary size, as those fixes become relatively intricate and I like them
less and less.

Each commit has its own improvement stats and mainly reduce generic-code
duplication through shared non-generic helpers and `#[inline(never)]`
boundaries.

Total reduction stats:

With sccache disabled, `cargo clean` and `cargo build --release -p zed`:
Before: zed binary 396716472 bytes
After: zed binary 359416504 bytes (-9.40%, -37.29MB)

Release Notes:

- N/A

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Co-authored-by: zed-zippy[bot] <234243425+zed-zippy[bot]@users.noreply.github.com>