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.
It does not. From what I could tell at the time, the `as` syntax was for casts and coercions were always implicit. In retrospect, I think I am wrong about that. I envisaged addressing this with type ascription. But I think that highlights some potentially confusing overlap between type ascription and explicit casts.