Rework parsing: More strict and (hopefully) cleaner

[Lingepumpe]

Reworked every parser file, with the exception of extensions.rs.

Changes and cleanup were mostly:

• Every parser is now responsible for consuming his own starting and closing tags (previously some parsers would be okay not having any starting tag, and would be used like this). To be able to achieve this any parser that calls further consume functions must use peek() to leave starting tags in the reader for the subparser to handle.
• Many parsers were accepting any amount of their starting tag, and also consuming any closing tag as valid, ignoring the name. Now exactly one opening tag and matching closing tag is required.
• Parsers that were already using peek() were doing so via a level of indirection through some form of "ParseEvent" enum, I think I have found a cleaner solution using .clone() on the event which allows us to remove this indirection and parse XmlEvents more directly even with peek().
• Added some more custom errors, and generally use custom errors more

[brendanashworth]

@Lingepumpe wow, thank you for the fantastic work you've put together. I really like what you've done with the parser. Just a few questions/suggestions about your implementation.

[Lingepumpe]

I believe I addressed all review concerns, or is there something else you would like me to fix? Or will some other person review as well? Just checking the ball is not in my court ;)

[brendanashworth]

@Lingepumpe sorry about the delay, I've been unable to compile your changes due to some issues with (presumably) my Rust setup, and I've also been very busy. I should have time to do a final review and merge this within the next two days, thanks for being patient. This looks really nice though.

[Lingepumpe]

It seems cargo fmt is now more restrictive with regards to "use": They are ordered in a certain way, and I also got more duplicate warnings. Updated the pull request to include those fixes.

[frewsxcv]

@Lingepumpe thanks for fixing the cargo fmt issue

@brendanashworth still planning to look at this? otherwise, i'm thinking about merging this tomorrow

[frewsxcv]

bors r+

[bors[bot]]

Build succeeded

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