Overhaul parsing from the presentation format #388
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- 'Tokenizer' works on UTF-8 strings rather than byte slices, so it does not have to perform any UTF-8 decoding itself. - 'Tokenizer' always provides entire tokens rather than individual symbols. Most of the time, symbols will be regular characters, so users shouldn't have to individually branch on each character. - Because the interpretation of escapes can vary depending on the semantic element (e.g. escaped vs. unescaped dots in domain names), tokens just provide the raw underlying string by default. Semantic element parsers are expected to validate tokens themselves, and will be able to use helper methods on 'Token' to process escapes.
This hopefully comprehensive algorithm deals with quoted strings and escaped characters therein as well as it can. Of particular note is the way it tries to insert newline tokens so that entries on different lines can be distinguished. It's difficult to do this for the last line of the file, so we don't try.
'Tokenizer::next()' is now an inherent method. It correctly increments the position counter after a token is output. It also ensures that no token contains invalid (non-printable) ASCII text. 'Token' now provides 'process()' which will handle escape sequences. For some elements (e.g. domain names), 'process()' is called _after_ some analysis of the raw text is performed. 'Scan' has now been rewritten in terms of 'Tokenizer'. The existing impls have been migrated over as well, and are much simpler. The next step is to update all the uses of the 'Scan' trait in other 'base' modules. After that, we can address 'zonefile'.
Given that the consumer function often just appends slices to a buffer, this version should be significantly more efficient. It re-uses slices of the original input in all cases except a '\DDD' escaped octet.
The major work here is implementing 'Scan' for absolute domain names. This integrates work from 'Name::from_symbols' and 'NameBuilder' into a single unified package, making it significantly easier to work with.
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