Remove comparator.hh and switch to <=> in a bunch of places

Known behavior changes:

- `MemorySourceAccessor`'s comparison operators no longer forget to
  compare the `SourceAccessor` base class.

Progress on #10832

What remains for that issue is hopefully much easier!

src/libexpr-c/nix_api_external.cc

@@ -115,7 +115,7 @@ public:
     /**
      * Compare to another value of the same type.
      */
-    virtual bool operator==(const ExternalValueBase & b) const override
+    virtual bool operator==(const ExternalValueBase & b) const noexcept override
     {
         if (!desc.equal) {
             return false;