Add AbstractBooleanDeclaredStatement/BaseBooleanStatementSupport
mandatory/config/require-instance/yin-element statements share the
same shape of being stateless boolean holders. Add an explicit
base class, making sure all common declarations end up referencing
the same 4 objects.
JIRA: YANGTOOLS-1065
Change-Id: I0c423ad1fc34c2a4c55c92d246830880a4c8bf9b
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit
7e5bb201aa0873de5a08707813cff7a24516d17f)
(cherry picked from commit
b91e27fedfa1ca36cfaef2762009fd5acb12dc59)