gh-124157: Lazily create __annotate__ functions for function annotations

[JelleZijlstra]

• Issue: PEP 649: Avoid creation of function objects for __annotate__ #124157

[JelleZijlstra]

@larryhastings I recall you were working on a version of this, but I got impatient and spent a bit of time on it this morning. This only covers annotations on functions, partly because I'm lazy and partly because those are by far the most common anyway. However, I ran some benchmarks and it's not clear to me this makes much of a dent on memory usage anyway; function objects aren't that big.