Tests for block from/to buffer roundtrip

Block serialization/de-serialization is a bunch of hand-crafted tedious code without tests (currently). It would be nice to have tests for these and might be an introduction for someone into the codebase and block hierarchy. Essentially the tests would create at least one variant of each kind of block and would try to convert it between high-level parsed types and underlying buffers, ensuring those operations are symmetrical.