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by POSIX. The other _t types that are not part of the API are still there
for now. Also, got rid of all that was left of the 64-bit types.

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.

copies of the FFT.

(sort of)

different precision for twiddles and data.

"inverse" flag from the state so it can be shared between the forward and
inverse transforms.