Rollup merge of #126830 - RalfJung:unsized-fn-params, r=compiler-errors

make unsized_fn_params an internal feature

As suggested [here](rust-lang/rust#123894 (comment)).
r? `@compiler-errors`

Fixes rust-lang/rust#123887 (kind of -- ICEs on internal features are considered acceptable so this issue is not-a-bug once this PR lands)