libsyntax: Accept `use foo as bar;` in lieu of `use bar as foo;`

The old syntax will be removed after a snapshot.

RFC #47.

Issue #16461.
auto merge of #16468 : pcwalton/rust/as-renaming-import, r=alexcrichton

The old syntax will be removed after a snapshot.

RFC #47.

Issue #16461.

r? @brson