Added flexible carets for `unexpected`/`fail`

[j-mie6]

Both fail and unexpected have rigid error carets, such that they can only be other like errors.

This PR introduces flexibility into these combinators: if the caret width is not manually specified, the caret is considered flexible, which means that it can be influenced by other errors on merge:

• a specialised error may borrow a wider caret from a vanilla error if they occur at the same offset and the specialised error's caret is flexible
• if a specialised error with a rigid caret is merged with a flexible caret, the rigid caret will always take precedence.

The rules are similar for unexpected, where a rigid caret replaces flexible ones. All relevant unexpected/fail based filtering combinators are defined rigidly, preserving their current semantics.

To recover the pre-4.2.11 behaviour on fail and unexpected, specify at caret width of 1 -- this change is added as these behaviours make much more sense, and the original behaviour is preservable.

[codeclimate[bot]]

Code Climate has analyzed commit a3b5875c and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 71.2% (50% is the threshold).

This pull request will bring the total coverage in the repository to 90.9% (0.0% change).

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