Fix <it> implementation to support @pos
Looks like there's a problem with <hi> tags.

    This constructs a unittest for real <hi> support.  However, snax
    isn't able to handle the recursive structures right now.
Update state machine to handle hi-loops.

    Unittest still doesn't pass -- I need to have a stack of tuv
    contents
Working <hi> implementation for one level of nesting

    This adds a content stack, which should be usable for <sub> as well.
    However, none of the tag stuff is currently nestable because the
    state handlers are singletons.  This means I will need to improve
    the stack to also capture tag content and make the handler stateless.

    Also, HiTag still needs to be removed from the tag hierarchy, as it
    doesn't support data.
Merge branch 'contentmodel'
Add a unittest (failing) for nested <hi> tags.
Move HiTag out of the InlineTag hierarchy - it doesn't support @data
Support recursive <hi> tags.