Harmonize `AsyncFn` implementations, make async closures conditionally impl `Fn*` traits
This PR implements several changes to the built-in and libcore-provided implementations of `Fn*` and `AsyncFn*` to address two problems:
1. async closures do not implement the `Fn*` family traits, leading to breakage: https://crater-reports.s3.amazonaws.com/pr-120361/index.html
2. *references* to async closures do not implement `AsyncFn*`, as a consequence of the existing blanket impls of the shape `AsyncFn for F where F: Fn, F::Output: Future`.
In order to fix (1.), we implement `Fn` traits appropriately for async closures. It turns out that async closures can:
* always implement `FnOnce`, meaning that they're drop-in compatible with `FnOnce`-bound combinators like `Option::map`.
* conditionally implement `Fn`/`FnMut` if they have no captures, which means that existing usages of async closures should *probably* work without breakage (crater checking this: https://github.com/rust-lang/rust/pull/120712#issuecomment-1930587805).
In order to fix (2.), we make all of the built-in callables implement `AsyncFn*` via built-in impls, and instead adjust the blanket impls for `AsyncFn*` provided by libcore to match the blanket impls for `Fn*`.
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