libsyntax: error on struct Foo {}.
auto merge of #5596 : luqmana/rust/unit-struct, r=catamorphism

Fixes #5449.
Fix warning about unused imports in import lists

Before, if anything in a list was used, the entire list was considered to be
used. This corrects this and also warns on a span of the actual unused import
instead of the entire list.
Removing unused imports