Unroll loops in s16n1s16 resampler
Unroll loops in s16n1s16 resampler

The most common 16-bit filter sizes are 16 and 24 samples, so we
completely unroll the inner loop.  The inner loop is only two or three
iterations in these cases.  Resulting speedup is 51-53% on test system.

depp/libfresample,depp/libfresample,h6ah4i/libfresample,h6ah4i/libfresample,depp/libfresample