Implement multidispatch and conditional dispatch. Because we do not
attempt to preserve crate concatenation, this is a backwards compatible
change.

Conflicts:
	src/librustc/middle/traits/select.rs
Add a model for how conditional trait impls might be used to implement
the Fn-FnMut-FnOnce hierarchy.
Convert tests to cross-crate, fix a RefCell bug I found in the process.
Add a few more debug statements
auto merge of #17669 : nikomatsakis/rust/multidispatch, r=pcwalton

Implement multidispatch and conditional dispatch. Because we do not attempt to preserve crate concatenation, this is a backwards compatible change. This is not yet fully integrated into method dispatch, so "UFCS"-style wrappers must be used to take advantage of the new features (see the run-pass tests).

cc #17307 (multidispatch)
cc #5527 (trait reform -- conditional dispatch)

Because we no longer preserve crate concatenability, this deviates slightly from what was specified in the RFC. The motivation for this change is described in [this blog post](http://smallcultfollowing.com/babysteps/blog/2014/09/30/multi-and-conditional-dispatch-in-traits/). I will post an amendment to the RFC in due course but do not anticipate great controversy on this point -- particularly as the RFCs more important features (e.g., conditional dispatch) just don't work without the change.