Remove identity_future indirection

This was previously needed because the indirection used to hide some unexplained lifetime errors, which it turned out were related to the `min_choice` algorithm.

Removing the indirection also solves a couple of cycle errors, large moves and makes async blocks support the `#[track_caller]` annotation.

tests/ui/async-await/track-caller/async-block.rs

@@ -1,9 +1,9 @@
 // edition:2021
 
-#![feature(closure_track_caller, stmt_expr_attributes)]
+#![feature(stmt_expr_attributes)]
 
 fn main() {
     let _ = #[track_caller] async {
-        //~^ ERROR attribute should be applied to a function definition [E0739]
+        //~^ ERROR `#[track_caller]` on closures is currently unstable [E0658]
     };
 }