Uncurried types: distinguish 0 args, and 1 arg of type unit.

[cristianoc]

Curried types () => int and unit => int are equivalent.
However, uncurried types have explicit arity. After this PR, (. ()) => int means arity 0, and (. unit) => int means arity 1. The distinction was always there, but it was not reflected in the syntax.
This is a breaking change. In addition, the formatter before this PR does not help, as it would always format to (. unit) => int.

In uncurried mode, these uncurried types are written as () => int and unit => int.

[mununki]

Not sure I understand correctly. After this PR, how can be matched between implementation and interface in curried mode?

// interface
(. unit) => int // arity 1

// implementation
(. ) => int // arity ?
(. ()) => int // arity 0