libcore: Move Cell to core and de-~mut core and std
test: De-~mut the test suite. rs=demuting
libsyntax: Stop parsing `~mut`
librustc: De-[mut] librustc (remove all mutable arrays). rs=demuting
test: De-[mut] (remove all mutable arrays from) the tests. rs=demuting
librustc: Make `&const [T]` mean the same thing as `&[const T]`