Add overloaded operators for ptr

[brson]

This seems like an obvious use case for operator overloading. Probably export them from core by default.

[eholk]

What operators did you have in mind? Things like + for ptr::offset?

[brson]

yes, + and -

[pcwalton]

Not going to work right now, because operators can't be unsafe. Not sure how to change that either.

Going to close for now.

[eholk]

At least for +, the operator could be "safe" with an unsafe block inside of it. You aren't going to have problems if you do pointer arithmetic until you dereference it, which would be unsafe anyway. In other words, if ptr::addr_of is safe (which is is), then it seems like pointer arithmetic is also safe.

[brson]

Yeah, that's true. We consider arithmetic safe, but indexing and dereferencing unsafe. Just having + would be convenient.

[MarkJr94]

I'm going to try on work on this. I'll add overloaded + and -. I'll use impl<T, I: Int> Add<I, *T> for T* as dbaupp suggested on IRC so that it will work with any primitive integer type.

[bstrie]

Looks like #7631 added + and - for unsafe pointers, can this be closed?