generalize state -- allow any state to be passed around

[mattfenwick]

Currently, a parser requires a rest and state parameter, and a parse result has a rest and state field.

These are both 'states' in the monad sense, and it might be useful for clients to have more state passed:

• parse input (string)
• parse position: index, line, column
• table of operator precedences -- modeling this as state allows it to be dynamically (contextually) updated

Also, note that the current state field is pretty exclusively used for position tracking, even though its name doesn't indicate that. That's confusing.