Tracking issue for `impl Trait` in `const` and `static` items and `let` bindings

[Centril]

As provided for by rust-lang/rfcs#2071, this is a tracking issue for impl Trait in:

• const items
• static items
• let bindings

Steps:

• Implement the RFC
  • In let: (Re-)Implement impl_trait_in_bindings #134185
• Adjust documentation (see instructions on rustc-guide)
• Stabilization PR (see instructions on rustc-guide)

Unresolved questions:

• Should the part about let bindings be recast more generally to be about coercions? This would allow foo as impl Debug and foo: impl Debug as well.

Blocking issues:

Incomplete. Search the F-impl_trait_in_bindings issue.

• impl_trait_in_bindings is a breaking change #83021
• impl_trait_in_bindings and pick-constraint region bounds #61773