syntax: add a visit_ident method to the Visitor.
Put non-ascii identifiers behind a feature gate.

cf. https://mail.mozilla.org/pipermail/rust-dev/2013-November/006920.html
auto merge of #10605 : huonw/rust/ascii-ident-gate, r=pcwalton

cf. https://mail.mozilla.org/pipermail/rust-dev/2013-November/006920.html