Define and implement proper representations for YANG List statement.

YANG list statement actually defines 3 behaviour models:

• Unordered Map
• Ordered Map
• List

Map behavior could be defined for lists which have key defined,
which add unique identifier to items.

Ordered behavior is defined by statement ordered-by.
When ordered-by has value "user" - insertion order
must be preserved, since ordering has semantic meaning.

List behaviour is list statement without key defined.