Improvements related to databake::test_bake!()

[alvra]

This PR improves the usefulness of the test_bake! macro by making sure the baked tokenstream matches the literal constructor.

• update the implementations for slices and tuples
• convert corresponding tests to use test_bake!
• enable some tests that now pass
• more test coverage for slices and Cow

I've also tried to modify the output of the Bake derive macro so test_bake! works everywhere but I couldn't figure out the logic for when commas are omitted or not so that's not part of this PR. Unfortunately this means that derived implementations still cannot always be tested using test_bake!. But with this PR, at least primitives, manual implementations, and combinations thereof will now work.

[CLAassistant]

All committers have signed the CLA.

[robertbastian]

Thanks, this is great! Did you check whether any other mentions of https://github.com/rust-lang/rust/issues/98906 in the repo can be reenabled now?

[sffc]

Does this also fix the other instances of rust-lang/rust#98906 scattered through the codebase?

[alvra]

I've enables 2 tests outside of databake that now pass.

• For one of these I had to update the test itself too, could you verify this is correct?
• There's another test ignored because of rust#98906 that fails for a different reason and could probably be enabled: components/list/src/provider/serde_dfa.rs:L231

The other 3 tests ignored use derived Bake impls so they still fail over commas unfortunately.

[Manishearth]

This is great, thanks!