implement `Default` for `AsciiChar`
This implements `Default` for `AsciiChar` in order to match `char`'s implementation.
From all the different possible ways to do this I think the clearest one is to have both `char` and `AsciiChar` impls together.
I've also updated the doc-comment of the default variant since rustdoc doesn't seem to indicate it otherwise. Probably the text could be improved, though. I couldn't find any similar examples in the codebase and suggestions are welcomed.
r? `@scottmcm`
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