Utilities & type traits (C++20)

Removes unnecessary decay uses in the library.

Forbids specializing standard function templates.

Adds is_nothrow_convertible.

Adds std::type_identity.

Fixes is_abstract/is_constructible for abstract class types.

Removes unnecessarily explicit default constructors from the library.

Adds unwrap_reference and unwrap_ref_decay.

Adds std::bind_front.

Allows reference_wrapper of incomplete types.

Makes variant and optional propagate copy/move triviality.

Sanitizes variant's converting constructor (no narrowing).

Adds visit<R>.

Makes std::function's move constructor noexcept.

Declares LWG 3016 not a defect and adjusts wording.

Improves the completeness requirements for type traits.

Makes underlying_type SFINAE-friendly.

Adds is_bounded_array and is_unbounded_array.

Adds is_layout_compatible, is_pointer_interconvertible_base_of and related functions.

Makes bind_front not unwrap reference_wrapper.