Also added helper macros for unpacking lists of variadic arguments.

Reimplement do_arith to take arguments this way.

Previously builtins could be called with something similar to:
     func(eval(args)...)
but in C++, the order of evaluation of the args is undefined.  R requires
that they are evaluated from left to right.

Improvements to arglist and promise handling

Rather than creating individual promise objects on the heap, the
data for default arguments is stored in a contiguous section in the
Frame object.  These are promoted to full promise objects only when
they are actually needed.

The remaining uses of Rf_matchArgs have been switched over
to ArgMatcher now and the code has been deleted.

- PromiseData objects can't be copied, only moved.
- A PromiseData can refer to an external Promise object.

R keeps a promised copy of the arguments passed to each closure call
to support functions such as nargs and UseMethod.
Moving them from the ClosureContext to the Frame reduces the amount of
code that deals with linked lists of promised args and makes it easier
to transition to more efficient representations.

Note that access to the promiseArgs should still go through the
ClosureContext object to ensure that the semantics are not changed.