Fix docs mismatch

[jakeheis]

This fixes the issue where Beak incorrectly matches up doc comments with functions when global variables are declared above the functions. A basic example of this issue can be seen with a Beak file such as:

public let str = String(repeating: " ", count: 4)

/// This is the first func
public func firstFunc() {

}

/// This is the second func
public func secondFunc() {

}

Beak incorrectly matches docs with functions as such:

 > beak list
Functions:

  firstFunc: This is the second func
  secondFunc:

My implementation of the fix, while I believe it is the most clean and simple, is not necessarily the most performant (given the index(where:) call inside the for loop). I thought it best to leave it simple for now, but I'm happy to optimize if you think it's necessary

[yonaskolb]

Good catch, nice one! I don't think the performance hits will hurt us in any significant way.

It would be great if SourceKitten was able to return Structure and Docs together, that way we wouldn't have to merge them. I'll open an issue there