resolve wasn't checking that a `use` referred to a public item.

r=brson

Forbid item macros from occuring where statement macros might occur, and allow item macros to look like statement/expr macros.

libsyntax: Remove the "by-mutable-ref" obsolete syntax error; it blocks useful function argument patterns. Add a test for the latter. r=brson

Conflicts:
	src/libstd/time.rs

Conflicts:
	src/compiletest/compiletest.rs
	src/libcargo/cargo.rs
	src/libcore/core.rs
	src/librustc/rustc.rs
	src/librustdoc/rustdoc.rc