Vectors don't allow a trailing comma

[msullivan]

If we are going to allow trailing commas for records and enums, we should probably allow them for vectors as well.

[bstrie]

I think this should technically be an RFC, but trailing commas in collections literals are really nice. I think someone (pcwalton?) was against them at some point, but I'd really love to see trailing commas supported across the board.

[nikomatsakis]

seeing as we support trailing commas in other contexts, I think this is more of a bug fix than a feature request and hence no need for an RFC.