Require qualifiers to be types.

And add a short-hand: a single letter type name now names a (single
letter) constant type, even without wrapping brackets (just as with
the empty constant.)

This is a reasonable compromise between convenience and
typo-protection since there aren't any non-constant types with one
letter names.

version_column is now a list instead of a colon-delimited string.  The
qualifier named in this list should be a type name, as opposed to the
yaml serialization of that type name.

So, this is correct:

```
version_column: [f, [version]]
```

This is not:

```
version_column: [f, "[version]"]
```