Makefile.PL: clean cover files and more
replace Urban vec by nvec (natural endianness)

see http://blogs.perl.org/users/rurban/2014/08/vec-is-slow-little-endian-and-limited.html
allow more options for t/0[0-2] tests (-urban -debug)
G and V indices can be higher than size, check min/max for needed bits.
the seed d can reach up to MAX_INT