[small_map] Use hidden friends for operator==/!=

[Quuxplusone]

clang -std=c++2b complains about the old way:

./src/base/small_map.h:555:40: warning: all paths through
this function will call itself [-Winfinite-recursion]
    const const_iterator& other) const {
                                       ^

There are two pieces to the "right" solution: First, use the hidden
friend idiom to make sure all our operator==s are symmetrical and
don't treat the first argument differently from the second. Second,
eliminate the heterogeneous comparison operators; just rely
on the existing implicit conversion from iterator to const_iterator
to do the right thing.

[Quuxplusone]

@OlafvdSpek ping!

[OlafvdSpek]

Pong!
Looks good to me.
Is the infinite recursion hit in practice though?

[Quuxplusone]

Is the infinite recursion hit in practice though?

Yes, certainly if the offending operator!= is ever actually called, it'll infinite-recurse. I don't know whether it's ever actually called anywhere within ctemplate, and I also don't know whether it's possible for the client-of-ctemplate to call it directly. But I can at least confirm that it's located somewhere visible enough to break the build of the client-of-ctemplate when they try to compile the header in C++20 mode. So it's probably also reachable at runtime somehow. (Otherwise, you should just delete the whole function as unreachable!)