Port to C++20 spaceship operators

[jwnimmer-tri]

This ports about 75% of our comparisons to be spelled using https://en.cppreference.com/w/cpp/language/operator_comparison#Three-way_comparison, focusing especially on the code in common.

While reviewing, it might help to remember how operator<=> works:

• When it is defaulted (= default), you get all 6 comparison functions (i.e., both equality and ordering).
• When it is defined by hand, you only get the 4 comparison functions (ordering) so still need to write operator== (equality) yourself to get the last two.

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This change is