Support syntax for named states

[mirisuzanne]

This maybe could have broken down into smaller PRs, but starting from 'named states', all these other changes seemed relevant.

• Add support for [list of named states]
• Switch root syntax to <states> at <initial>
• Support additional 'overflow' options: cycle, cycle-on, sticky
• Switch trigger syntax to next | prev | set <state>
• Define how named/integer states 'match'
• Allow out-of-bounds states to be set explicitly, and define increment/decrement behavior

I'm not sure I made all the best choices in terms of where different aspects of the logic are defined. But it's a draft…