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 would be a bug, so should not be user visible. But point taken about jargon. Using `unexpected type in enc_sty (ty_open)`, which tells us something different, but hopefully more useful with less jargon (even if totally opaque to users).