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/pos-idx.hh

@@ -28,20 +28,15 @@ public:
         return id > 0;
     }
 
-    bool operator<(const PosIdx other) const
+    auto operator<=>(const PosIdx other) const
     {
-        return id < other.id;
+        return id <=> other.id;
     }
 
     bool operator==(const PosIdx other) const
     {
         return id == other.id;
     }
-
-    bool operator!=(const PosIdx other) const
-    {
-        return id != other.id;
-    }
 };
 
 inline PosIdx noPos = {};