Merge BuiltIn resolution and KnowledgeBase match steps into a single step about ergo HOT 3 CLOSED

This isn't as easy as I thought, but exploring the problem is yielding some optimizations already.

from ergo.

It turns out that the semantics of builtins are kinda similar to lambdas. Further investigation warranted

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In the end I ended up adding a Maybe<BuiltIn> to predicates, so that they can be stored in the knowledge base as usual.

This somewhat broke one use case for libraries, which was to provide a different set of builtins depending on the interpreter scope. Now the builtins are baked into the knowledge base and some hoops need to be jumped through to update it dynamically after creation, especially because the static analyzer needs to be notified of these changes. This leads to #75.

But aside from that, this effectively solves the issue and removes the ambiguity between builtins and regular predicates, with the exception that builtins have extremely granular control over how they are compiled.

from ergo.