Relax Logos::Error bounds

[DavidCrossman]

Motivation

Logos::Error has several trait bounds which are not necessarily required. These extra bounds impose restrictions on the custom error type the user provides. In particular, the Default bound may require adding a #[default] Other variant to an error enum even if an error callback is specified.

Relaxing the Default bound

There are three cases to consider:

1. The user does not supply a custom error type
2. The user supplies a custom error type with an error callback
3. The user supplies a custom error type without an error callback

For (1), the fallback error type is (), which implements Default, so the bound is irrelevant.
For (2), the user's error callback is called rather than Default::default, so the bound isn't needed.
For (3), the _make_error function will fallback to Default::default. If the custom error type doesn't implement Default, this will cause a type error. The user gets an error message suggesting to implement Default on the custom error type.

This makes the Default bound required only if the user does not provide an error callback (case (3)), rather than unconditionally. The Logos derive already uses this technique to require Into<CustomErrorType> for each callback's error type. Relaxing the Default bound in this way would be backwards compatible.

Examples and Book

I've updated the book to reflect these changes. I have also simplified the examples which use error callbacks.

Relation to #459

This PR encompasses #459, so it could be merged before this PR (or closed).

[codspeed-hq]

Merging this PR will not alter performance

✅ 6 untouched benchmarks

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_{Comparing DavidCrossman:relax_error_bounds (67e4554) with master (d5fea38)}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.66%. Comparing base (b6dc674) to head (67e4554).
⚠️ Report is 11 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #570   +/-   ##
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  Coverage   73.66%   73.66%           
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  Files          29       29           
  Lines        1929     1929           
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  Hits         1421     1421           
  Misses        508      508           

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