DST coercions and DST fields in structs

The commits are not quite stand alone, I should probably squash them together before landing. In particular if you review the individual commits, then you'll see some scrappy stuff that gets fixed in later commits. But reading the commits in order might be easier to get an overall idea of what is going on.

The first commit includes putting back time zone into our time library - @pcwalton removed that as part of his de-~str'ing, but I had already converted it to use StrBuf, so we may as well leave it in. Update: no longer, this is removed in a later commit.
Yeah, that is right. Unifying ty_str etc., at the moment breaks the world because we have loads of impls for both `&T` and `&[T]`. But once the trait reform lands, that should no longer be a problem. Even without trait reform, with complete DST we should be able to fix all those impls.