Signed overflow

correct_fma still act properly with internal overflow
dec inc  still wrap and next and prev can have the expected behaviour
multiplies is amended in order that no overflow will be produced on unsigned short products but wrapping as expected
sqrs and negs have proper scalar version to avoid computation that can lead to overflow
the integer test for pow was not coorrect
nan case is fixed in scalar for expm1

all unit tests pass on x96 sse4.2 with my clang

cover an exhaustive are not tested yet
I don't think that's a good idea. It's up to the user not to call dec with entries that overflow.