of 32-bit in find_spectral_pitch()

This involved adding kfft_single.[ch] that redefines kiss_fft a second time
with a different prefix. All this is still a bit of a mess now. The mask
had to be converted to 16-bit input, but we're still using floats to apply it.

handling of the left side slope of the spreading function (right-side coef
was used)

(sort of)

masking computation functions.

float)

copyright, so it's fine!)

to make the code more C89-friendly.

pre-computed (instead of computing tons of pow() uselessly all the time)

do anything yet.

Code much leaner now.