Implement REXP classes for special environments
Collect serializations of special environments
Add test cases for parsing of special envronments

Note: QAP protocol serializes special environments with XT_UNKNOWN,
so these cases will fail in QAP-related tests and are marked as
SKIP. This is tracked separately in issue #33.
Implement parsing of special environments (closes #31)

Special SEXPTYPE markers 241 (BASEENV_SXP) and 242 (EMPTYENV_SXP)