compile-time _chk: specialize str{cat,cpy}_s, memcpy_s

skip run-time checks when already checked at compile-time.
split functions into two: unchecked and checked, depending
on the already done compile-time check.

currently only useful for clang-5+
gcc cannot not handle BOS as compile-time constant, how lame.

shaves off 10% p_memcpy_s, when all 4 args are of known size + constant len.
See #48