Removes `__continuation`, `__await`, and `__run`.
In part here, the discussion was that while the feature had been useful for validating the early explorer design, it's no longer needed for that role as the explorer is now quite robust. Continuations have been experimental and, at this point, don't have an owner pushing to a proposal.
The triggering factor is that, as we push to address fuzzer issues, I ran into a crash bug in this code; basically, `fn Main() -> i32 { __await; return 0; }`. When I mentioned this, the reaction seemed to trend towards removal of the feature.
This is intended to be used for template instantiation, but for now takes no stance as to what it's cloning.
This is tested by parsing and cloning all of our test files, and checking that the result of converting each AST to proto is the same as the result of cloning and then converting each AST to proto.
This is in preparation for template instantiation being triggered during substitution, and being able to fail.
Fix rule-of-three violation (missing assignment operator) in `Error` that got in the way of using it to hold an error temporarily in a failed transformation.
These are used by the AST in lots of ways, and this resolves various layering issues.
This means that `AllocationId` also lives in ast/, but is managed by interpreter/. A better layering here would be desirable, but this seems good enough for the time being.
Jon Ross-Perkins
Richard Smith