Introduce application syntax for ADTs

[yannham]

Now that typechecking, pattern matching and core contracts are implemented for ADTs (enum variants), we switch from the temporary to the final syntax, which is just to use the same application syntax as for function application.

There was an unforeseen ambiguity: function argument patterns can be chained for multi-ary function, for example fun {x,y} {z} => x + y + z. This would clash with unparenthesized enum variants: fun 'Foo 'Bar => could be either fun ('Foo) ('Bar) or fun ('Foo 'Bar).

This PR chooses the first one: for a top-level function pattern argument, we only allow parenthesized enum variants such as fun ('Foo 'Bar). Thus fun 'Foo 'Bar => is necessarily fun ('Foo) ('Bar). The rationale being that it's more in line with other patterns, where each top-level space delimit function arguments, as in fun x y, fun {x} {y}, etc.

Doing so, we apply the same restriction to nested enum variants, to forbid patterns like let 'Foo 'Bar 'Baz = x, which could be parsed unambiguously as 'Foo ('Bar 'Baz), but are quite confusing.