bf57e8ada6
Add a builtin `FnPtr` trait that is implemented for all function pointers r? `@ghost` Rebased version of https://github.com/rust-lang/rust/pull/99531 (plus adjustments mentioned in the PR). If perf is happy with this version, I would like to land it, even if the diagnostics fix in 9df8e1befb5031a5bf9d8dfe25170620642d3c59 only works for `FnPtr` specifically, and does not generally improve blanket impls.