Maybe make params a class

[nworden]

Hiroshi brought up that it'd be good to make env a class, and that was done in PR #629. Maybe it'd be a good idea to do something similar with params. It'd probably be simpler with params since they're "dumb" -- there's not any real handler logic involved in setting the params, so the class' constructor could just be passed the request object and set itself up.

As with env, if we made params a class we could reduce the risk of typos, because each parameter name could be defined as a property on the class. (TBH, what made me think of this was pylint complaining that Structs don't have members for various parameters we access as fields of the params variable.)