Mapping comparison operators are not constrained properly

[dalg24]

For instance with layout_right, the standard says

template<class OtherExtents>
  friend constexpr bool operator==(const mapping& x, const mapping<OtherExtents>& y) noexcept;

Constraints: extents_type​::​rank() == OtherExtents​::​rank() is true.
Effects: Equivalent to: return x.extents() == y.extents();

and our implementation does

mdspan/include/experimental/__p0009_bits/layout_left.hpp

Lines 190 to 194 in 26f4d3a

	template<class OtherExtents>
	MDSPAN_INLINE_FUNCTION
	friend constexpr bool operator==(mapping const& lhs, mapping<OtherExtents> const& rhs) noexcept {
	return lhs.extents() == rhs.extents();
	}

I realized that as I was reviewing Chrisitan's tests for libcxx