Support variables with known value(s)

Parameters can now have a known value that affects the CostEstimator but won't be affected by constant folding. I didn't add code to actually provide known values in this pull request, since it seems like that would be more tied into a different Trello card (https://trello.com/c/Src9XXxU/533-delay-prepared-statement-planning-until-an-initial-set-of-parameter-bindings-is-available).