fix(semantics): support asIteration() for _iter nodes

[DonIsaac]

What This PR Does

Adds support for the builtin asIteration() semantic operation to _iter nodes.

Before, calling someIterNode.asIteration() would throw this error:

missingSemanticAction: Missing semantic action for '_iter' in operation 'asIteration'.
NOTE: as of Ohm v16, there is no default action for iteration nodes — see 
  https://ohmjs.org/d/dsa for details.

Now, asIteration() will just return the _iter node it was called on.

Why?

I often found myself switching between +/* and ListOf while writing grammars. This allows semantics to continue to work as intended without changes.