Haskell-like list notation

[rolyp]

Reinstate Haskell-style list notation, perhaps as syntactic sugar for (:) and Nil. There is a precedent for omitting it (PureScript), but given our in-progress notation for list comprehensions (#277), it seems natural to include it.

• parse Nil as []
• add (empty/non-empty) list notation to surface Expr
• corresponding expr desugaring rule from formalism
• add (empty/non-empty) list notation to surface Pattern
• corresponding pattern desugaring rule from formalism
• parse non-empty list notation in expressions
• parse non-empty list notation in patterns
• ban direct use of Nil, Cons in surface language – for consistency with Pair