[reg] isNaN deprecation for integers

Here we go, back and forth :)

Same as https://github.com/D-Programming-Language/phobos/pull/2311 but defines new overload
as deprecated and does not add tests / documentation for it.

Also includes https://github.com/D-Programming-Language/phobos/pull/2372

Can anyone hint what other functions have been moved to std.math and thus are affected by same issue?