Better pretty-printing

[pieter-bos]

The current printer was invented ad-hoc and does not work very well: it contains a state machine with 200 lines of definitions and still does not print whitespace correctly. I think this is a promising method for pretty-printing. We should adapt it a bit for our case:

• I think it's still desireable to implement syntax for the different frontend languages, without redefining the shared syntax for each language. We should have a way to reflect this in the generated tree. Should it be a case in Doc?
• I want to be able to track that a particular Doc is for a particular Node, perhaps we can just have a case for a non-layout-affecting node :: Node -> Doc -> Doc. This would hopefully allow us to print [---- / ----] bars around a node within a printed AST.
• We should have the ability to step in and out of ghost code. This may only be discovered late in generation, e.g. //@ ghost x = 1 + \old(x) should be printed in ghost again, but only because \old appears in it: assignment and addition are of course supported outside of ghost code. Alternatively we could ignore this entirely and insist that the AST indicates whether something is ghost or not structurally.

[pieter-bos]

• The language is part of the context with which nodes are rendered
• NodeDoc surrounds every node, guarded via some nice protected[this] trickery
• Ghost is essentially ignored, except for statements and structural nodes like contracts that are obviously ghost. We should implement a case class GhostStatement(inner: Statement) or so in the future, similar for ghost declarations.