harden typed_python serialization and allow message size limits

[braxtonmckee]

right now if you try to send a very large string (say, 100gb), the deserializer will allocate space for it. We should allow clients to impose strict limits to ensure they don't blow up on bad data, and should attempt to agressively fuzztest sending bad/corrupted data and verifying that we get reasonable exceptions instead of segfaults.