Replace overload constraints with static if

[wilzbach]

From a recent NG thread by @WalterBright:

Also, as mentioned in the std.algorithm.mutation.remove case, constraints in Phobos often confuse "requirements" with "specializations".
Requirements should be user-facing constraints, while specializations are implementation details better handled with internal static if.

This PR picks a couple of low-hanging fruits in std.algorithm. If this PR got too large, let me know.

[JackStouffer]

Yeah, this is a huge diff. If it's not too much work, could you please split it into multiple commits?

[quickfur]

Yes, please split into multiple PRs. It's hard to review a diff this large.

[wilzbach]

Yeah, this is a huge diff.

Oh sorry. It didn't look to me like this at the time.

If it's not too much work, could you please split it into multiple commits?

Not at all - it's automated