[stepchowfun]

This ports the changes to error.rs from Mull, along with the line_index.rs it now depends on. Both files now match Mull's byte for byte, except for Mull's quick fixes (the Fix type, the fix field and accessor, and the corresponding constructor argument).

• Each error's listing finds its lines with a LineIndex instead of scanning the source from its start, so reporting many errors no longer takes time proportional to the number of errors times the size of the source. Error::new's source context is now (&str, &LineIndex, SourceRange).
• format_errors appends each error to the output instead of rebuilding it, which was quadratic in the number of errors.
• A listing no longer panics when a range ends within a line's indentation, excludes indentation from every line's highlight, and no longer ends with a row of only spaces when its last line has nothing highlighted.

Each phase which reports errors builds a LineIndex once at its entry point and passes it along when constructing errors, so the many tests that call these phases don't change.

The only visible change in the output is that an error at the end of a file is followed by an empty line rather than a line of spaces.

Status: Ready

Fixes: N/A