Ensure we only serialize valid pseudotypes

[danielmewes]

In issue #2399 a table became inaccessible because a r.literal() pseudo-type made it to disk. When reading the value back, an exception was thrown and the deserialization of the datum_t failed.

We should add a verification to datum_t's serialization code to make sure that illegal pseudotypes are not written to disk.
What makes this slightly more complicated is that the question of whether a given r.literal is legal or not depends on the context of the serialization. It is never legal to write one to disk, while it is sometimes legal to serialize it for a network message.