multiple-item traversal ranges

[dsimcha]

== Quote from spir (denis.spir at gmail.com)'s article
> On 03/30/2011 05:03 PM, Andrej Mitrovic wrote:
> > Doesn't opApply provide this?
> Yes, but using opApply seems discouraged (scheduled for deprecation?).

opApply is less composable, and in current implementations, less efficient than
ranges.  However, it is **_NOT_** going anywhere, since both internal and external
iteration have their advantages.  opApply and the tradeoff between internal and
external iteration are documented in TDPL.  An example of this is that iterating
over trees is hard to express with ranges, because it requires an explicit stack.
 With opApply it can be written as recursion.

> And
> opApply doesn't allow playing with other parts of "new style D" (esp algorithms).

IMHO it should where possible, but a few bugs in opApply prevent me from fixing
this.  Phobos should recognize "Iterables", or any type that can be iterated with
a single-argument foreach.  Input ranges are a subtype of iterables.  Eventually
std.algorithm should treat iterables as a supertype of ranges and things like map
and filter should work.