Rollup merge of #143640 - oli-obk:const-fn-traits, r=compiler-errors

Constify `Fn*` traits

r? `@compiler-errors` `@fee1-dead`

this should unlock a few things. A few `const_closures` tests have broken even more than before, but that feature is marked as incomplete anyway

cc rust-lang/rust#67792

Author: matthiaskrgr