PrimeCandidateSource: two extra query functions.

pcs_get_upper_bound lets the holder of a PrimeCandidateSource ask what
is the largest value it might ever generate. pcs_get_bits_remaining
lets it ask how much extra entropy it's going to generate on top of
the requirements that have already been input into it.

Both of these will be needed by the upcoming provable-prime work to
decide what sizes of subsidiary prime to generate.