Rollup merge of #119222 - eholk:into-async-iterator, r=compiler-errors,dtolnay

Add `IntoAsyncIterator`

This introduces the `IntoAsyncIterator` trait and uses it in the desugaring of the unstable `for await` loop syntax. This is mostly added for symmetry with `Iterator` and `IntoIterator`.

r? `@compiler-errors`

cc `@rust-lang/libs-api,` `@rust-lang/wg-async`

Author: compiler-errors