[key reuse] refactor & unify key reuse rule registration

[jakevdp]

This refactoring will pave the way for making key reuse checking part of the core dispatch path. There are two goals:

1. Move from having two tables of key reuse rules (static and dynamic) to having a single table: this will let us begin easily specifying key reuse signatures at the location where primitives are defined.
2. Move shared code into single utility functions that can be referenced from the main dispatch path: this will let us move away from the current approach which involves monkey-patching the impl rules.

[froystig]

It seems that we could unify KeyReuseSignature and DynamicKeyReuseSignature by treating all of them as functions of some number of operands in the equation, where KeyReuseSignatures are specifically nullary. Is that worth reflecting in the code (making everything one type, and always a function)?