Verbose and flexible args and kwargs syntax

[Eelco]

To get back on topic a little bit, lets get back to the syntax of all
this: I think we all agree that recycling the function call syntax is
less than ideal, since while it works in special contexts like a
function signature, its symmetric counterpart inside a function call
already has the meaning of a function call.

In general, we face the problem of specifying metadata about a
variable, or a limited form of type constraint.

What we want is similar to function annotations in python 3; in line
with that, we could have more general variable annotations. With an
important conceptual distinction; function annotations are meaningless
to python, but the annotations I have in mind should modify semantics
directly. However, its still conceptually close enough that we might
want to use the colon syntax here too. To distinguish it from function
annotations, we could use a double colon (double colon is an
annotation with non-void semantics; quite a simple rule); or to
maintain an historic link with the existing packing/unpacking syntax,
we could look at an augmented form of the asteriks notation.

For instance:

def func(list*args, dict*kwargs) <- list-of-args, dict-of-kwargs
def func(args::list, kwargs::dict) <- I like the readability of this
one even better; args-list and kwargs-dict

And:

head, deque*tail = somedeque
head, tail::deque = somedeque

Or some variants thereof