Text_document.offset and Text_document.offsets

[ddickstein]

We should be able to avoid exposing the zipper in Text_document.Expert by adding the following functions to the non-expert API:

val offset : t -> Position.t -> int
val offsets : t -> Range.t -> int * int
val position : t -> offset:int -> Position.t
val range : t -> start_offset_inclusive:int -> end_offset_exclusive:int -> Range.t

The positions should be interpreted with the text document's encoding.

[ddickstein]

(The latter two obviate the recently added advance function)