filterx keys function

[bshifter]

example:

            d = {"foo":1,"bar":[3,2,1],"baz":{"tik":"tak"}};
            $MSG.keys = keys(d); #["foo","bar","baz"]
            $MSG.elt = keys(d)[0]; #"foo"

[bazsi]

Just one minor comment, also I feel increasingly that the function support code needs to be improved, there's still to much boiler plate around creating a function and writing its argument parsing.

In a similar vein, I'd want to turn FilterXFunction as an expr into a FilterXCall, which operates on various function types, e.g. simple and generator and generic, each with more supporting framework to help facilitate the writing of those functions.

But that's just a side note, this one can go in even with the extra argument check. I'd appreciate the removal of all similar checks as a separate PR though.

[alltilla]

@bshifter Can you please create a newsfile entry for this in another PR, thanks!