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.
Is there an issue open somewhere for adding support for unifying `[T]` with `U`?

I didn't see one when skimming the list on the #12938 metabug.

Might be good to open an issue and put it on the metabug and this comment.