Constant bindings

[comnik]

We should add support for constant bindings in ...

• predicates
• function expressions
• rule expressions

For predicates and function expressions support is rather straight-forward, but for rules we can spend some more time thinking about how to do it efficiently. A first idea was to turn constant bindings to rule inputs into predicates that are then pushed down as far as possible into the rule body itself, to avoid unnecessary materialziation of tuples.

[bachdavi]

• Add possible explicit :ValueType

For example unix timestamps (> ?t {:Instant 132352525}) for things that cannot be inferred.