rename the Ptr trait to RawPtr

Closes #6607
auto merge of #6913 : thestinger/rust/ptr, r=graydon

Closes #6607

I went with `RawPtr` instead of `UnsafePtr` because not all of these operations are `unsafe`, so to me it makes more sense to refer to it as a "raw" (not wrapped/abstracted) pointer. If we decide on something else in #6608 it can be renamed again.