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.
I'll admit it, I'm not willing to spend the time figuring out what is being accomplished by having two `as` expressions above, since as far as I can tell, the `_` should be inferred to be `A` anyway.

Please do either or both of the following:
 * add a comment explaining why you need two `as` expressions, or
 * plug in the actual type distinct from `A` into the underscore.