Add DataTreeAwareEffectiveStatement

Add DataTreeAwareEffectiveStatement

YANG defines two distinct ways to look at a YANG model:
- schema tree: The definition hierarchy specified within a module.
- data tree: An instantiated tree of any data modeled with YANG,
             e.g., configuration data, state data, combined
             configuration and state data, RPC or action input,
             RPC or action output, or notification.

This means that there really are two notions of 'a child node',
based on schema tree, or based on data tree. The data tree is
a strict subset of schema tree. The layout of the data tree is
such that it does not include RPCs, actions nor notifications --
addressing data tree nodes encapsulated in these requires
first reaching the encapsulation point via schema tree walk.

This patch introduces DataTreeAwareEffectiveStatement
and its companion DataTreeEffectiveStatement, updating extant
statements to support it.

Construction of this namespace has a side-effect of finding
duplicate data nodes within each level of hierarchy, thus fixing
YANGTOOLS-883.

JIRA: YANGTOOLS-853
JIRA: YANGTOOLS-883
Change-Id: Id02bb2557845000fe7fa90d1bf56c8425cce24fb
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
Signed-off-by: Jie Han <han.jie@zte.com.cn>