Proposal for nicer init()

Through trial and error I got following to pass:

```
tmp = ObjCClass("UINavigationController").alloc().initWithRootViewController_(root)
```

Wouldn't it be nice to get that data from call signature:

```
tmp = ObjCClass("UINavigationController").alloc().init(rootViewController=root)
```

And then, perhaps, move to:

```
UINavigationController = ObjCClass("UINavigationController")
tmp = UINavigationController(rootViewController=root)
```

Start by reading the current ObjCClass and allocation/init entry points, then reproduce the UINavigationController examples from the issue. Done means the proposed keyword-based init behavior is specified and implemented consistently, including the shorter UINavigationController(rootViewController=root) form, with tests covering the supported call signatures.

Written by the indexing model from the issue text.