Cleanup fmaf() usage, move to lmms_math.h

Per request, moving fmaf() to lmms_math.h for cleaner code.

I originally planned on using a macro, but we can hit two birds and just do manual fma in the same function if we declare it as a function.  I'm not sure if this has any performance hits from a compiler perspective or even if it's desired, so @diizy, please advise. :)

Also, I've chosed the camel-case names `fastFma()`, `fastFmal()`, `fastFmaf()` to be consistent with the nomenclature `fastPow()` in `lmms_math.h`, however [`fast_rand()`](https://github.com/LMMS/lmms/blob/stable-1.1/include/lmms_math.h#L126) should then be renamed `fastRand()`, but I find that change out of the scope of this change. :)