convert (usize, usize) for byte offsets into Range<usize>

[QuietMisdreavus]

Right now, anything that gives text spans gives offsets as a tuple. This requires any substring access to go through a cumbersome text[range.0..range.1] to get the substring. Using std::ops::Range for these would greatly simplify getting these substrings.

[QuietMisdreavus]

Bah, never mind, Range isn't Copy. This would require cloning the Range out to prevent moving out of the struct, which is just as awkward.

Potential solution: functions that subslice the text for you, or generate the Range that you can then tweak. If the range is stored alongside the text, this could work, but I think Entities aren't, so we're back to the tuple/range conversion.