std: Add init and uninit to mem. Replace direct intrinsic usage
std: Add move_val_init to mem. Replace direct intrinsic usage
std: Move byteswap functions to mem
std: Make mem's doc slightly more accurate
auto merge of #12124 : brson/rust/intrinsics, r=thestinger

As mentioned https://github.com/mozilla/rust/pull/11956#issuecomment-34561655 I've taken some of the most commonly-used intrinsics and put them in a more logical place, reduced the amount of code looking in `unstable::intrinsics`.

r? @thestinger