support infer()

this is a first cut at supporting inference of RHSs

CandidateMap.h and .cpp became basically empty as a result of this change so I eliminated them

there are some name changes that I didn't do, in interest of keeping the size of the change smallish, such as changing the CandidateMap into perhaps Candidates or CandidateList or something

also there may be some files that change name, but again I wanted to keep the diff manageable

Priority is gone, replaced by some code that counts dups in SolveCandidateMap()
This change comes about as a consequence of other changes. Here is the command that is being run for this test:

```
/home/regehr/f/souper-regehr/build/souper /mnt/ssd/f/souper-regehr/build/test/Extractor/Output/illegal-sext.ll.tmp | /home/regehr/f/souper-regehr/build/parser-test
```

Souper is calling SolveCandidateMap which, lacking a solver, is just dumping replacements, which at that point contain only the LHS. To get the RHS I can replicate the true/false guessing logic (as I did in one other place) or else we can dump the replacements from somewhere lower in the stack where the RHS is available.