Get rid of structural records in libsyntax and the last bit in librustc.
librustc: Separate the rest of the trait bounds with `+` and stop parsing space-separated ones. rs=plussing
auto merge of #5069 : pcwalton/rust/plussing-2, r=pcwalton
auto merge of #5071 : luqmana/rust/derec, r=pcwalton

Rid libsyntax of records and get rid of the last piece in `librustc/front/test.rs`.