syntax: remove dead @mut Visitor impl (only used in tests).

Also, move some code only uses in the tests into the test module, and
replace some needless @mut ~[] with plain ~[].
Handle another possible error in a unix pipe test

This cropped up on the bsd bot, and if it's an error that gets thrown then it's
fine to just whitelist another type of error in the test.
add support for the `cold` function attribute

This allows a function to marked as infrequently called, resulting in
any branch calling it to be considered colder.
Register new snapshots
auto merge of #10133 : alexcrichton/rust/another-error, r=thestinger

This cropped up on the bsd bot, and if it's an error that gets thrown then it's
fine to just whitelist another type of error in the test.
auto merge of #10117 : huonw/rust/dead-visits, r=sanxiyn

Used nowhere, and these are likely incorrect anyway: self needs to be
dereferenced once more otherwise the method calls will be reusing the
current impl... bam! Infinite recursion.
auto merge of #10127 : thestinger/rust/cold, r=pcwalton

This allows a function to marked as infrequently called, resulting in
any branch calling it to be considered colder.
auto merge of #10135 : alexcrichton/rust/snapshots, r=thestinger

Plus some migration from `let mut this = self` to `mut self` (yay!)