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.

r? @brson
Could we retain the parsing but emit a warning for awhile instead of removing this entirely after a snapshot lands? I'd just think the comment should be rephrased to say something like "make this a warning after a snapshot".

Also, could you make sure that the string "stage0" shows up here? (useful for grepping)