Layout generation for recursive lambda sets

[ayazhafiz]

Adds support for recursive and nullable-unwrapped lambda sets, so things like this work now:

» compose = \f, g -> \x -> g (f x)
…
… identity = \x -> x
… exclaim = \s -> "\(s)!"
… whisper = \s -> "(\(s))"
…
… res: Str -> Str
… res = List.walk [ exclaim, whisper ] identity compose
… res "hello"

"(hello!)" : Str

Closes #3444

[folkertdev]

Nice, a major missing lambda set feature!