<compare> implement == for comparison categories (#242)

WG21-P1614 "The Mothership has Landed" added `==` operators to the comparison category types (`weak_equality`, `strong_equality`, `partial_ordering`, `weak_ordering`, and `strong_ordering`) defined in `<compare>`. This PR implements those operators to bring the comparison category types up to spec once again. It also implements P1614R2's removal of operators that rewrite into calls to `operator==(X, nullptr_t)` for each comparison category type `X`.

Drive-by changes:
* Move the TODO list of tasks for WG21-P0768 "Library Support for the Spaceship (Comparison) Operator" completion out of `<compare>` and into #64
* Remove the `#if 0 // Not yet implemented` block from `<compare>`