add uncheckedptr<T> type constructor

[dtarditi]

Greg Morrisett suggested that we add an unsafe_ptr constructor that is an abbreviation for T *. I think this is a good idea because it will allow code to be parameterized by the kind of pointer. This will be useful in C when using macros and likely useful when extending Checked C to C++.

[gdr-at-ms]

There are lot of T*s in C; I would think unsafeptr<T> is a bit too broad, IMHO.

In the C++ core guidelines we added owner<T> as alias for T* to indicate a specific set of problematic situation.

[dtarditi]

We discussed this and decided that unchecked_ptr would be a better name. Gabby was somewhat concerned about unsafe_ptr because there are several sources of unsafety, and currently we are addressing only one of them.

We like the idea, but do have some concerns about how it might interact with other uses of type constructors to denote properties in C++. We are going to wait to add this until we have some motivating examples gathered from experience using or applying Checked C.