Avoid using memcpy on STRSXPs

As per correspondence with Matt Dowle, SET_STRING_ELT ensures that
objects are properly aged (see CHECK_OLD_TO_NEW in SET_STRING_ELT,
in memory.c). If we just use memcpy, because CHARSXPs are SEXPs,
there is a chance they will be gc'ed if R needs to shrink the
global string cache, and a raw memcpy won't properly protect them
from the gc.