src/{fvec,fmat}.c: use memcpy and memset to optimise when possible, add option to disable
src/cvec.h: improve cvec_t api
src/pitch/pitchyinfft.c: beautify frequency weighting coefficients
src/fmat.c: use memset for each columns
src/lvec.c: no need to set to 0 after calloc
src/{fvec,cvec,lvec,fmat}.c: make sure new_ functions return NULL if length <= 0