[cbor] Implement `JsonGenerator.writeArray()` methods to be added in 2.8

[cowtowncoder]

As soon as support for efficient scalar-array writes in jackson-core (see FasterXML/jackson-core#277) is added, should add specialized implementation for cbor backend, since this allows use of length-prefix notation, needed/preferred by some decoders.

Also related to #1 and #2.