Implement dynamic (modifier + clause)

Fixes #51, but since it was really a missing feature and not a bug, I
put the test in the main tests instead of the issues.
Fix #52: throw; (without expression)
Fix bug with specifier tokens

I’m not really confident this is the final solution, but it fixes one
problem in the SDK.
Implement attribute (named) arguments

Found in Ceylon SDK:

    value something = MyClass {
        attr => attr;
    }
Implement AttributeSetterDeclarations

    assign myAttribute {
        // ...
    }

How did I never implement this‽

(spotted in Ceylon SDK)
Implement KeyValueIterators

    for (key->item in something) { ... }

(spotted in Ceylon SDK)
Fix mixed wildcards and imports

To rename certain elements, it can actually make sense to import both
particular members and everything else with a wildcard:

    import ceylon.process {
        stdin=currentInput,
        stdout=currentOutput,
        stderr=currentError,
        ...
    }

(spotted in Ceylon SDK)
Implement Package (subclass of Atom)

Apparently, you can use package.identifier to use the top-level
declaration instead of the one of your local scope. Learned something
new today!

(spotted in Ceylon SDK)
Implement type parameters

(spotted in Ceylon SDK)
(but let’s pretend that this is such an important language feature that I
 couldn’t possibly have missed it for six months. Please?)
Implement Supertype

(spotted in Ceylon SDK)