Allow Rete to inline helper/flatten patterns

[bergmanngabor]

Rete does not do query flattening by default. There should be an opt-in way to specifically request certain helper patterns to be flattened (inlined) into calling patterns. For instance, this could apply to patterns marked as @Inline.

The main expected benefit is code conciseness - the ability to factor out helper patterns that are not enumerable (e.g. the whole pattern is just an eval). Reusable patterns currently have to be stand-alone queries with enumerable result sets. LS can do without this restriction, if certain parameters are marked as in parameters - with the guarantee that matches are enumerable for each specific binding of in-parameters. Rete can not handle parameter directions and will complain that a certain pattern body is not enumerable. A fully general solution would employ magic sets to ensure that Rete only computes results for specific combinations of in parameters. However, this is fairly involved improvement. A much simpler low-hanging-fruit solution (though perhaps less efficient for more complex helper patterns) would be inlining.

A secondary, minor improvement may be performance - maybe we can do with one fewer copy of the match set of the helper pattern if it is directly inlined.