Simpler class instantiation

[madsmtm]

Swift decided to combine allocation and initialization, such that you can simply call NSView.init() instead of Objective-C's cumbersome [[NSView alloc] init]. Actually, NSView.alloc() is entirely impossible!

For us, this means:

• NSView::initWithFrame(mtm, frame) (or (frame, mtm), depending on what we decide in The deal with mutability #265) instead of NSView::initWithFrame(mtm.alloc(), frame).
• NSString::initWithString(string) instead of NSString::initWithString(NSString::alloc(), string).

Should we do this? What would be the downsides? And how should this tie into our naming scheme in #284, since Rust prefers the name new?

Relates a bit to #438, since we would like the syntax for declaring initializers to resemble the syntax actually used when calling them.

[madsmtm]

One downside would be that all superclasses would have to manually create initializer methods inherited on superclasses instead of e.g. doing Id::cast::<MySubclass>(MySuperclass::init(MySubclass::alloc().into_super())).

Though that should arguably be discouraged anyhow, and since we already generate the initializer methods from superclasses on subclasses in header-translator, this is more of a problem for downstream users.

[madsmtm]

I'm not going to go down this route, as I think what the user declares in declare_class! should match what's in extern_methods!, and making declared initializers not take this: Allocated<Self> is going to feel inconsistent and weird.

Things will get better here anyhow after rust-lang/rfcs#3519, which will allow us to write self: Allocated<Self>, and call with NSView::alloc_on_main(mtm).initWithFrame(frame) / NSString::alloc().initWithString(string).