Built-in JSON data type.

Like the XML data type, we simply store JSON data as text, after checking
that it is valid.  More complex operations such as canonicalization and
comparison may come later, but this is enough for not.

There are a few open issues here, such as whether we should attempt to
detect UTF-8 surrogate pairs represented as \uXXXX\uYYYY, but this gets
the basic framework in place.