Fact-binding in :or conditions

[alex-dixon]

• Investigate whether Clara supports this
• If Clara supports it, investigate changing our syntax. Boolean conditions presume a positional tuple expression or another boolean expression, both of which are represented as a single vector. For fact-binding support within them we might need to enforce double vector syntax for a positional tuple match expression.
• Throw a syntax error when trying to use a fact binding within an :or expression, especially if this turns out to be "illegal" or something we won't allow in the foreseeable future. Currently we return the fact binding as an eid match expression, which yields incorrect results.