Fix for #flat_map

[soutaro]

Enumerable#flat_map haven't type checked as expected in Steep.

# def flat_map: [A] () { (Element) -> (A | Array[A]) } -> Array[A]

[1,2,3].flat_map {|x| [x.to_s] }          # Steep infers Array[Array[String]] while we need Array[String] here

This is because the subtyping relationship Array[String] <: (A | Array[A]) resolves to a substitution of A == Array[String]. If it resolves to a substitution of A == String, the type checking will work.

So, we'd like to introduce a function #hole_path. It takes two arguments, type and type variables, and returns the shortest path to unknown variable. Giving higher precedence to types with longer path will result more specific (smaller) type variable assignment.

This depends on two intuitions (without any proof):

1. Specific type is better (A == String is better than A == Array[String])
2. Longer #hole_path implies more specific type

Fingers crossed that this gives a better result.