Remove resolve's xray infrastructure

Since the removal of privacy from resolve, this flag is no longer necessary to
get the test runner working. All of the privacy checks are bypassed by a special
item attribute in the privacy visitor.

Closes #4947
Move std::rt::io to std::io
Improve error message for breaks in blocks

Before it was always stated that it was a "break outside of a loop" when you
could very well be in a loop, but just in a block instead.

Closes #3064
auto merge of #10423 : alexcrichton/rust/move-io, r=pcwalton

I still plan on reorganizing the internals of the IO module a bit, but this is the major move which has been on the horizon for awhile.
auto merge of #10427 : alexcrichton/rust/no-xray, r=brson

Just a few triage issues that I ran into.