libsyntax: Remove all non-`proc` `do` syntax.
librustc: Remove non-procedure uses of `do` from librustc, librustdoc,
and librustpkg.
test: Remove non-procedure uses of `do` from compiletest, libstd tests,
compile-fail tests, run-fail tests, and run-pass tests.
test: Remove all remaining non-procedure uses of `do`.