Fixed bugs where pretty-printer generated invalid Elm code

I noticed a couple of bugs where the generated pretty-print of an AST doesn't compile back to Elm code. In particular, tuples were being generated with _TupleXX as a constructor, where XX is the number of elements. Also, some patterns were being improperly parenthesized. For example, (\[x,y] -> x + y) was being printed as (\x::y::[] -> x + y) instead of (\(x::y::[]) -> x + y).