auto merge of #7471 : msullivan/rust/default-methods, r=graydon

This fixes a bunch of default method bugs and restructures how vtable resolutions are represented.

(It also adds a depth counter to llvm::type_to_str as a hacky work around for our circular llvm types. This is related in the sense that I needed to do it to make debug tracing not cause rustc to crash after running out of stack space.)