Accept trailing commas in struct patterns

We decided in the 12/10/13 weekly meeting that trailing commas should be
accepted pretty much anywhere. They are currently not allowed in struct
patterns, and this commit adds support for that.

Closes #10392
auto merge of #11075 : alexcrichton/rust/issue-10392, r=brson

We decided in the 12/10/13 weekly meeting that trailing commas should be
accepted pretty much anywhere. They are currently not allowed in struct
patterns, and this commit adds support for that.

Closes #10392