Rollup merge of #48130 - ollie27:stab, r=Mark-Simulacrum

Correct a few stability attributes

* `core_float_bits`, `duration_core`, `path_component_asref`, and `repr_align` were stabalized in 1.25.0 not 1.24.0.
* Impls for `NonNull` involving unstable things should remain unstable.
* `Duration` should remain stable since 1.3.0 so it appears correctly in the `std` docs.
* `cursor_mut_vec` is an impl on only stable things so should be marked stable.

diff --cc src/libsyntax/feature_gate.rs
Simple merge