Replaces the memcpy() and memmove() with std::copy_n() and std::copy_backward() in fastserializer.cxx. Change-Id: I6d971f18d1138f03ab62a5007db5d0da63f7679a Reviewed-on: https://gerrit.libreoffice.org/c/core/+/203377 Tested-by: Jenkins Reviewed-by: Jonathan Clark <jonathan@libreoffice.org>