Give Attributes more attributes for padding data.
Rewrite tag parser to be cleaner and safer.

All tag tests passing. Still need to finish backslash support and
support for templates and tags within <open> tags.
Add docstrings for a couple new methods in the tokenizer.
Add a number of new tag tests.
Pass some tests by simplifying the way tags are read from the stack.

Two still fail because templates aren't implemented yet, but those
are otherwise handled correctly.
Update the integration.rich_tags test to use the new tag tokens.

Remove an now-unused import in the tokenizer.
Support templates and wikilinks inside <open> tags (part 1)
Support templates and wikilinks inside <open> tags (part 2)
Replace data.literal and data.quoted with a data.CX_QUOTED context
Support tag nesting properly; unit tests; recursion checks for tags.
Refactor more of the tag tokenization process.
More tag refactoring; fix some bugs.
Add three tests for invalid attribute quote usage.
Change 'write' to 'emit'; adjust some other names for PEP8.