Stabilize fn-like proc macros in expression, pattern and statement positions I.e. all the positions in which stable `macro_rules` macros are supported. Depends on https://github.com/rust-lang/rust/pull/68716 ("Stabilize `Span::mixed_site`"). cc https://github.com/rust-lang/rust/issues/54727 cc https://github.com/rust-lang/rust/issues/54727#issuecomment-580647446 Stabilization report: https://github.com/rust-lang/rust/pull/68717#issuecomment-623197503.