refactor: Use comparison operator in comparison overloads

[rprospero]

C++20 added the comparison operator (sometimes referred to as the spaceship operator), from which all of the other comparison operators can be derived. I've used this to cut down on a bunch of the boiler plate in our iterator overloads and for our Number type.

There's one change in the handling of NaN for the Number class. For any floating point value x, the following statements are always true under the new setup, but were always false under the old setup:

a < NaN
Nan < a

This was done to allow strong ordering of Number, despite floating point values only having weak ordering. This convention was chosen to preserve the the following three relations, which I considered more fundamental:

a != NaN
NaN != a
NaN != Nan