stl_bind: add binding for std::vector::clear (#2074)

diff --git a/tests/test_stl_binders.py b/tests/test_stl_binders.py
index b83a587..c7b7e85 100644
--- a/tests/test_stl_binders.py
+++ b/tests/test_stl_binders.py
@@ -64,6 +64,9 @@
     del v_int2[-1]
     assert v_int2 == m.VectorInt([0, 99, 2, 3, 4, 5, 6, 7, 0, 1, 2, 3, 88])
 
+    v_int2.clear()
+    assert len(v_int2) == 0
+
 # related to the PyPy's buffer protocol.
 @pytest.unsupported_on_pypy
 def test_vector_buffer():