Only Barf sizeof ConsCell* pointer bytes from car_/cdr_.

Also, re-enable symbol related ReaderTests.
Implement bool operators for ConsCells.

Re-enable the rest of the ReaderTests.
Add HackToFixNil.
Only init kNil/kT once in Reader::Init, and call HackToFixNil if needed.
Update REPL to read/print a ConsCell * rather than a Sexp&.