Correct the type ID for lliby_{true,false}_value
Flesh out the runtime infrastructure

- Add GarbageState enum to define the garbage collection state contained
  in BoxedDatum::gcState

- Add PairValue implementation. This is aliased to Cons for use in the
  allocator

- Add allocateCons/freeCons that are just wrappers around malloc/free at
  the moment. This will allocate from our semispace later

- Add custom new operator for BoxedDatum that wraps allocateCons.
  Explicitly delete the delete operator
Fix Scheme serialization of AbstractSyntaxTree

This extends the SchemeParserSuite to use a new method called
assertReflexiveParse. First, it parses the passed string and verifies it
matches the expected value. Then it serializes the parsed data back to a
Scheme string and re-parses it. Finally it confirms the re-parsed data
still matches the expected value.

assertReflexiveParse caught bugs in the serialization of reals, symbols
and characters. This fixes all found issues.