- Fix a panic when a 0 length span covers the end of a document
- Fix incorrect `line_count`
- Add new unit tests and complete existing ones
- Improve readability

…ent line" and "just the span"

Fixes zkat#355

Change the number of context lines from usize to Option<usize> to allow
choosing between "just the span" (as implemented previously when using
0 context line) using `None`, and displaying the error line without
extra context (not possible before) using `Some(0)`.

- Mark as dead_code the structs uses in testing
- Remove unused code inherited from Eyre for converting `Option` into
  `Result`

A zero-length span at a line boundary can be associated either with the
previous or the next line. Current code prefer to use the next line,
however there isn't always a "next line" (end of source, or "no context"
configuration).
There is also the extra-special case of an empty document which has no
"next line" but also no "previous line".

This commit adds an empty newline at the end of a document if appropriate
so that there is a "next line", or use the "previous line"