Investigate getting rid of the AsyncFn* family of traits in favor of LendingFn*

[compiler-errors]

This will allow us to avoid annoying trait duplication if we want to add gen || and async gen || closures.

We need to lower impl async Fn() bounds to LendingFn bounds using the associated_type_bounds feature. This doesn't work currently until:

• For a rigid projection, recursively look at the self type's item bounds to fix the associated_type_bounds feature #120584 fixes associated type bounds in some positions
• We fix hrtb + infer types break auto traits with return type notation  #109924 (well, the general problem of HRTB with projections that have infer vars in them)
• Figure out if we want to add a new associated type to Fn/FnMut, or just have two new traits, i.e. figure out how to unify the fact that LendingFnMut doesn't necessarily have FnOnce as a supertrait.