Three-way comparison (<=>)

Adds <compare> library support for the spaceship operator.

Makes <=> symmetric for rewritten expressions.

Consistency improvements for <=> and other comparison operators.

Separates == from <=>: defaulted <=> implies a defaulted ==, and != is rewritten from ==.

Defines when <=> is used for defaulted comparisons of members without <=>.

Adds <=> across the standard library ('the mothership').

Tunes up spaceship: rewritten candidates and defaulted comparison rules.

Makes <compare> freestanding.

Allows defaulted comparison operators taking parameters by value.

Removes weak_equality and strong_equality.

Cleans up the specification of defaulted comparisons.

Allows defaulting comparison operators outside the class.