Make bits serialization sorted (by position)

Yang specification describes canonical form for bits to be ordered by position: http://tools.ietf.org/html/rfc6020#section-9.7.3

But yang-data-api specifies:
public interface BitsCodec<T> extends Codec<T, Set<String>>{
and Set loses the ordering.
This makes ODL report bits in pseudorandom order, which is not nice as users need more logic to compare outputs, or convert them to canonical form if needed.

getBits() of BitsTypeDefinition already is a List, so it can preserve given ordering. But BitsCodes API change is required to make the ordering visible for serialization. Also, BitsTypeBuilder.build() is already aware of positions, but it does not use this information when passing arguments to constructor of BaseBitsType (usinging positionMap as a TreeMap could fix that).

As this improvement requires an API change, it could be done as part of Binding Specification v2 initiative.