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.

src/libstd/io/cursor.rs

@@ -296,7 +296,7 @@ impl<'a> Write for Cursor<&'a mut [u8]> {
     fn flush(&mut self) -> io::Result<()> { Ok(()) }
 }
 
-#[unstable(feature = "cursor_mut_vec", issue = "30132")]
+#[stable(feature = "cursor_mut_vec", since = "1.25.0")]
 impl<'a> Write for Cursor<&'a mut Vec<u8>> {
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
         vec_write(&mut self.pos, self.inner, buf)

src/libstd/path.rs

@@ -576,7 +576,7 @@ impl<'a> AsRef<OsStr> for Component<'a> {
     }
 }
 
-#[stable(feature = "path_component_asref", since = "1.24.0")]
+#[stable(feature = "path_component_asref", since = "1.25.0")]
 impl<'a> AsRef<Path> for Component<'a> {
     fn as_ref(&self) -> &Path {
         self.as_os_str().as_ref()