Add support for slices

[espindola]

LLVM uses a lot of StringRef and ArrayRef. They are really handy and efficient, being implemented with just a pointer and size.

Some uses of it are safe (call a function with StringRef to a string that outlives the call), some unsafe (StringMap return a StringRef).

Go makes this kind of things builtin to the language (http://golang.org/doc/effective_go.html#slices), maybe we could do the same.

[graydon]

If this emerges it'll come from the work on regions, and be implementable in libraries. Closing as indefinitely-postponed for now. It's unlikely this bug will remain relevant when the pieces are in place.