Auto merge of #142289 - fmease:maybe-perf-gen-args, r=compiler-errors

[perf] `GenericArgs`-related: Change asserts to debug asserts & use more slice interning over iterable interning

1. The 1st commit yields the following perf gains: [#142289 (comment)](rust-lang/rust#142289 (comment)).
2. The 2nd commit might also have a minor positive perf impact, however that one wasn't tested in isolation.

For reference, the initial approach rust-lang/rust@c7e6acc (results: rust-lang/rust#142289 (comment)) had a lot more changes (apart from what's now contained in commit 1 and 2) which seemed to be perf irrelevant (cf. the partial countercheck in rust-lang/rust@6f82bf1 (results: rust-lang/rust#142289 (comment)).

Author: bors