Consider renaming trait `Ptr` to `UnsafePtr`

[brson]

It's only used for unsafe things. When I saw it I thought it may be implemented by other types.

It doesn't help that core::ptr deals with both unsafe and borrowed pointers.

[graydon]

did we not decide to use "raw" for these things?

[graydon]

accepted for backwards-compatible milestone

[thestinger]

@graydon: #6608 is related, we haven't really settled on unsafe vs. raw across the codebase

[thestinger]

Fixed by #6913.