Changing metatable contents

[Mark Hamburg <mhamburg@...>]

We've been looking at some data loading issues on my team, and the issue
that you shouldn't change metatable contents (or at least not which keys are
present) has come up. Is there a reason for this admonition other than an
artifact of the implementation? Could it be removed by clearing the fasttm
cache flags whenever we set an entry in a table?

Mark