Remove redundant import error
Fix failing test
actually fix failing test
auto merge of #6400 : cmr/rust/remove_useless_import_error, r=thestinger

Every unresolved import is reported. An additional error message isn't useful
and obscures (imo) the real errors: I need to take it into account when
looking at the error count.
Add vec.rs each2_mut function
Add vec.rs each2_mut testsuite