Tests for the FlatBuffers API

[ryzhyk]

No description provided.

[ryzhyk]

Some additional test dimensions:

• Move type declarations into a module to test for namespace-related issues
• Bit vectors >128 bit wide
• Wrap various types in type aliases
• Test user-defined generic types
• Error handling (not just for flatbuf-related stuff):
  • commit/rollback without start should throw DDlogException
  • writing invalid DDlogRecord or DDlogRecord that does not match relation type should throw DDlogException
  • using invalid relation id in a command should throw DDlogException
  • inserting duplicate key or deleting non-existing key in a relation with primary key should throw DDlogException
  • attempt to reuse the same UpdateBuilder for more than one update should throw DDlogException
  • error messages should correctly identify the problem in each case

[mihaibudiu]

I can work on some of these.

[ryzhyk]

Thanks, that would be great! I'll push a PR soon and will switch to writing documentation. You can pick it up from there.