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 --combined src/libsyntax/feature_gate.rs
index ea916d5168c335afa289e36f5f923500bc82a1b9,f8dbc4d0f4532bdcb920e437754439c104aca45d..3b137f9570a390f138b9e9540acc19bd34ba13e5
--- 1/src/libsyntax/feature_gate.rs
--- 2/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@@ -446,9 -446,6 +446,9 @@@ declare_features! 
  
      // Allows `#[repr(transparent)]` attribute on newtype structs
      (active, repr_transparent, "1.25.0", Some(43036)),
 +
 +    // Use `?` as the Kleene "at most one" operator
 +    (active, macro_at_most_once_rep, "1.25.0", Some(48075)),
  );
  
  declare_features! (
@@@ -541,7 -538,7 +541,7 @@@ declare_features! 
      // instead of just the platforms on which it is the C ABI
      (accepted, abi_sysv64, "1.24.0", Some(36167)),
      // Allows `repr(align(16))` struct attribute (RFC 1358)
-     (accepted, repr_align, "1.24.0", Some(33626)),
+     (accepted, repr_align, "1.25.0", Some(33626)),
      // allow '|' at beginning of match arms (RFC 1925)
      (accepted, match_beginning_vert, "1.25.0", Some(44101)),
      // Nested groups in `use` (RFC 2128)
@@@ -987,11 -984,6 +987,11 @@@ pub const BUILTIN_ATTRIBUTES: &'static 
                                   "wasm_import_memory attribute is currently unstable",
                                   cfg_fn!(wasm_import_memory))),
  
 +    ("rustc_args_required_const", Whitelisted, Gated(Stability::Unstable,
 +                                 "rustc_attrs",
 +                                 "never will be stable",
 +                                 cfg_fn!(rustc_attrs))),
 +
      // Crate level attributes
      ("crate_name", CrateLevel, Ungated),
      ("crate_type", CrateLevel, Ungated),
@@@ -1261,9 -1253,6 +1261,9 @@@ pub const EXPLAIN_PLACEMENT_IN: &'stati
  pub const EXPLAIN_UNSIZED_TUPLE_COERCION: &'static str =
      "Unsized tuple coercion is not stable enough for use and is subject to change";
  
 +pub const EXPLAIN_MACRO_AT_MOST_ONCE_REP: &'static str =
 +    "Using the `?` macro Kleene operator for \"at most one\" repetition is unstable";
 +
  struct PostExpansionVisitor<'a> {
      context: &'a Context<'a>,
  }