Defer copy decisions to StatementSupport

Defer copy decisions to StatementSupport

Core reactor should have no knowledge of specific statements, but
rather should provide general facilities for statements to decide
what they need to do. A typical example of this are
description/reference/status statements, which are never inlined
from a grouping parent (as they relate to the grouping, not the
copy target).

Add StatementSupport.copyAsChildOf() and implement above case
in terms of the new method.

Furthermore move support code to UsesStatementSupport and make it
use the same mechanics -- eliminating NOCOPY_FROM_GROUPING_SET
in the process of doing so.

JIRA: YANGTOOLS-694
Change-Id: I446266249dfe3f1bcac310bd16aece5185b40fb6
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>