Implement p_firsym.
Implement p_firsym.

Implementation is vectorized along the input samples 'x' as in p_fir. The length of the full filter is constrained to be odd, i.e., p_firsym only implements FIR filters of type I.

Signed-off-by: Chamon <88a261efe38359fe9d3c657d10e85c86b766fe68@gmail.com>

parallella/pal,eliteraspberries/pal,olajep/pal,eliteraspberries/pal,eliteraspberries/pal,debug-de-su-ka/pal,parallella/pal,olajep/pal,8l/pal,parallella/pal,mateunho/pal,aolofsson/pal,aolofsson/pal,Adamszk/pal3,parallella/pal,debug-de-su-ka/pal,debug-de-su-ka/pal,mateunho/pal,mateunho/pal,debug-de-su-ka/pal,aolofsson/pal,Adamszk/pal3,debug-de-su-ka/pal,Adamszk/pal3,8l/pal,mateunho/pal,olajep/pal,mateunho/pal,Adamszk/pal3,eliteraspberries/pal,eliteraspberries/pal,aolofsson/pal,parallella/pal,8l/pal,olajep/pal,8l/pal