Take a position on positions (and spans, and highlights, etc)

[kjosib]

Rather than making every application worry about every aspect of position tracking, the library should handle more of the job.

The next step is to define a standard Span type, a few supporting operations (including integration with failureprone::class SourceText), and some standard practices which go into a doc page. Longer term, perhaps a next generation of the system could do more automatically.

Implementation note: Things are probably simpler, faster, and more robust if spans are half-open ranges of positions rather than anchor-width pairs.

[kjosib]

The position on spans is the "slice" type.