Integer partition generator

[I've written an integer partition generator](https://github.com/bow-swift/bow/blob/master/Tests/BowGenerators/Data/Tree%2BGen.swift#L31). For example, I have used it to write a generator for a tree:
```swift
class Tree<A> {
    let root: A
    let subForest: [Tree<A>]
}
```

Let me know if you consider that this belongs to SwiftCheck, I'll then open a PR.