Reuse effective substatement lists when possible

This is the first step in fly-weight statements. As described in YANGTOOLS-1067, we want to reuse List<EffectiveStatement> instances if they do not change during the copy operation.

This is mostly yang-parser-reactor work, but also requires exposing access to AbstractStatementSupport.createEffective(Current, List).

This should prove effective in this test case:

module foo {
  namespace foo;
  prefix foo;

  grouping grp {
    container foo {
      description "desc";
      reference "ref";
    }
  }

  uses foo;
}

while /grp/foo and /foo are distinct objects, their EffectiveStatement.effectiveSubstatements() need to return the same object.