Hotfix for stmtsep use in YangParser grammar.

The stmtsep can occour 0..n times. Based on that the use of stmtsep? was replaced for stmtsep*
Added definitions for use of extensions outside of body statements in ext-use.yang.

Change-Id: I23092a7a6a5474bfed214ad08f64346a17b13df3
Signed-off-by: Lukas Sedlak <lsedlak@cisco.com>
Merge "Hotfix for stmtsep use in YangParser grammar."