Refactor `CastInterface` and `Cast`

[rodber]

The concept of CastInterface is misleading as what this package does is type checking against dynamic variables. Cast is often understood as type casting/conversion, which is not what is happening here. The name was poorly chosen.

What Cast does is that it takes a variable and it offers methods to access that variable typed:

public function int(): int {
    return $this->var;
}

^ This is what Cast does, it uses this trick to enable you to dynamic typed access.

As this is not converting anything, we need a new name for the next major.

I suggest TypedInterface.

$typed->int();