Const-qualify operator()s. NFC. (#1638)

As a general rule, `operator()` should be const-qualified.
The exceptions are few and far between.

I was actually looking for a Guideline on that subject;
I grepped for `operator()` and found that not only is there
no such Guideline yet, the doc actually contained these
places that (needlessly) violated the general rule.

CppCoreGuidelines.md

@@ -14523,7 +14523,7 @@ It is harder to ensure absence of errors in detached threads (and potentially de
     void f() { std::cout << "Hello "; }
 
     struct F {
-        void operator()() { std::cout << "parallel world "; }
+        void operator()() const { std::cout << "parallel world "; }
     };
 
     int main()
@@ -14537,7 +14537,7 @@ It is harder to ensure absence of errors in detached threads (and potentially de
     void f() { std::cout << "Hello "; }
 
     struct F {
-        void operator()() { std::cout << "parallel world "; }
+        void operator()() const { std::cout << "parallel world "; }
     };
 
     int main()