Don't store Compare in Classifier + more tidbits.

[Morwenn]

This commit ensures that a comparison function is always passed to every function, but never explicitly stored. It seems to generate a better assembly output.

There are several other minor things: less template parameters, more local type aliases instead. I also removed the explicit passing of template parameters to functions since they were not needed. That, and I turned the std::enable_if_t into static assertions that should give more understandable error messages.

[lorenzhs]

Thanks – in the future, could you keep commits as a logical unit (in this case, one commit for the Compare stuff and one for the templates)? That makes it easier to review and understand (and revert if something breaks).

[Morwenn]

Yeah, I only locally reworked the compare branch at first, changing stuff on-the-fly to check, but I was a bit too lazy to roll all the extras back. I generally do try to keep commits as logical units, but laziness won yesterday, sorry .___.

[lorenzhs]

No worries ;)