AST-12 add debug_show

[nomeata]

This is essentially #171, but

• rebased onto current master
• not using a Show unary operator any more, but rather a separate AST node
• using IR.flavor to track that we eliminate ShowE in the Show pass correctly.
• support for showing variants added.

We can change or remove this feature later, but right now @matthewhammer was pretty desperate not having to write debug printing code by hand.