show methods for trees / Node

[mortenpi]

There are different representations of Node object that are useful in different cases:

1. Something short that just says that this object is a Node with some element (current behavior).

2. Full AST printout (the current showast function, replicating the input of the @ast macro). This is useful when working with the tree manually, but is technical and can get pretty long.

   This can maybe be combined with (1), in that for large printouts we just put an ellipsis like we do for large arrays.

3. Pretty-printed document (the behaviour of CommonMark). This is useful for users who do not want to be concerned with the technical details of the representation, and also relevant for e.g. rendering docstrings.

We need to decide on which one should be the default output if a Node is returned in the REPL, and how to access the other option.