MultiPatterns

[philzook58]

Multipatterns are multiple possibly disjoint patterns to seek out in a search process. They are distinct from guards in that they may bind new variables. This pull request attempts to implement somewhat efficient multipatterns in a backwards compatible way.
The pattern compiler can remain mostly as is. Compiler state about what patterns are where in the subst map needs to be retained between each single pattern compilation.
Multipatterns require changes to the abstract machine because scanning for roots of the 2nd pattern onwards becomes required.
Extending the interface to be able to give pattern names to the root of the pattern itself enables significant extra modelling power. Note that labelled multipatterns allows one to implement efficient manual scheduling of search even for single patterns via flattening or breaking the pattern trees up.

This PR is still a work in progress, but examples can be found in the datalog.rs test. The syntax for creating multipatterns might change, but it looks this currently:

    let rules = vec![
        multi_rewrite!("base-case"; "?x = true = (edge ?a ?b)" => "?x = (path ?a ?b)"),
        multi_rewrite!("transitive"; "?x = true = (path ?a ?b) = (edge ?b ?c)" => "?x = (path ?a ?c)"),
    ];

Closes #39

[philzook58]

In the right hand side, I think I can only express equality to a variable bound in the lhs. In the Applier impl for MultiPattern, I would think one should allow creating new bindings only in the ?x = _ variable position to give name to new terms constructed. See test unbound_rhs. I understand there is something suspicious about allowing new bindings in the rhs, but I genuinely think this is useful and enhances expressivity. I should probably come up with a less contrived example though.
I have slightly changed how Applier works to allow this.

[philzook58]

This is an unfortunate clone, but I don't see an elegant way around it without changing interface of matches to be &mut or something.

[mwillsey]

This was merged, but I guess the fast-forward make github not see it.