Auto merge of #108080 - oli-obk:FnPtr-trait, r=lcnr

Add a builtin `FnPtr` trait that is implemented for all function pointers

r? `@ghost`

Rebased version of rust-lang/rust#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.

Author: bors