[cbor] Fail to report error for trying to write field name outside Object (root level)

[cowtowncoder]

(note: same as FasterXML/jackson-core#282)

Current check for writeFieldName() does not prevent write at root level (or possibly even in array?), and happily writes a text value. This should not be allowed as it is likely to result in invalid output.

Note: same problem occurs with CBOR and Smile as well; possibly with other format backends as well.