This change allows users to insert pre-encoded JSON `iodata()` anywhere
there can be a valid `json_value()` (i.e., anywhere except object keys).
This approach occurred to me looking at PR #139 from @dhull. The
technical difference being that this approach does not copy the given
`iodata()` into the output and instead re-uses the same input term as
part of the output `iodata()`.