[rust] add a converter from concrete nodes to generic Node

[froydnj]

The motivating example for this change is a visitor that wants to maintain parent node information as it traverses the AST. The type of the stack maintained during the traversal should be Vec<Node>, but there's no way from the concrete nodes to upcast to the Node enum.

[HParker]

This pattern is also common in the C code, so makes sense you sometimes want a generic node.

[HParker]

@froydnj What do you think about a test here as well?

[froydnj]

@HParker done! (Mostly just to make sure that as_node() compiles correctly.)

[HParker]

Mind squashing into logical commits as well? Then I think we are good to merge.

[kddnewton]

Yeah just squash and then we can merge

[froydnj]

Squashed, thanks!