Allow IR nodes to retain the originating CST node

[ggiraldez]

Follow-up from #1266, from these review comments:

By default, choice types and unique language terminals don't have an associated CST node in the IR nodes.

I imagine we will need the CST ref/location for very few nodes in the tree (mainly expressions).

I wonder if this should opt-in instead? something like model.retain_cst_node(identifier)...

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I think some of the "unique" terminals in choices will need a ref to their CST node. For example: keywords that push literal values (true / false / etc...).

I wonder if this should also be opt-in instead? i.e. model.retain_cst_node(identifier), which will generate a sub-sequence for it:

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And also, the association is through the node ID, but we may as well save the Rc<>:

Q: I wonder why is this a raw node ID rather than the node RC directly?

For linked non-terminal nodes this would make the default Debug implementation of IR nodes unusable for debugging, since they will recursively print the children nodes. We can override the Debug trait though.