ReplacementContext (was PrintContext) is produced by the LHS parser
and printer, and consumed by the RHS parser and printer; it ensures
that the mapping between names and pointers is consistent across
multiple conversions between Souper's internal and serialized
representations

also, making print names for vars optional and off by default
Merge pull request #111 from regehr/printcontext

PrintContext is produced by the LHS parser and printer, and