Eliminate wasteful PoR public-input conversions.

[porcuquine]

Avoid needless intermediate conversions to/from Vec<bool> when converting usize to Vec<bool>. Current public input representation for the PoR circuit trivially encodes challenges as the corresponding Fr, so nothing more is required.

NOTE: when we do pack more efficiently, it will not be necessary to restore the logic replaced here. Then we can use bit-shifting (<<) and bitwise or (|) (or +) to keep input generation cheap. [EDIT: We will not be able to use a simple conversion from u64 then — and the try_from used here is meant as a reminder not to. Instead, we will need a way to assemble an appropriate FrRepr directly. Alternately, we can trade some performance for simplicity and build up the result with Fr operations on individual challenges.]

Hat tip to @simonatsn and @sean-sn for observing the waste.