Use new stateful filter API for filtfilt #102
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This provides reduces the memory allocation and provides a small speed boost (on 0.4; on 0.3, where SubArrays are slow, it's probably substantially slower). We could conceivably use this to provide in-place filtfilt, although that is not yet implemented.
I removed support for non-1 initial values of a in filt_stepstate, since we don't need it if the filter is normalized before it is used.