Naming in range.ml

[mikeshulman]

I am never sure whether to open Asai.Range or not. On one hand, it contains functions and types that it would be convenient to be able to refer to unqualified, like located and locate. But on the other hand, it contains the type t which is clearly intended to be used qualified, and I'd rather not pollute my root namespace with a type called t.

[favonia]

I think it's good to think about this issue when (re)designing locations and diagnostics (#184 and #169).