Make PYBIND11_OBJECT_CVT only convert if the type check fails

Currently types that are capable of conversion always call their convert
function when invoked with a `py::object` which is actually the correct
type.  This means that code such as `py::cast<py::list>(obj)` and
`py::list l(obj.attr("list"))` make copies, which was an oversight
rather than an intentional feature.

While at first glance there might be something behind having
`py::list(obj)` make a copy (as it would in Python), this would be
inconsistent when you dig a little deeper because `py::list(l)`
*doesn't* make a copy for an existing `py::list l`, and having an
inconsistency within C++ would be worse than a C++ <-> Python
inconsistency.

It is possible to get around the copying using a
`reinterpret_borrow<list>(o)` (and this commit fixes one place, in
`embed.h`, that does so), but that seems a misuse of
`reinterpret_borrow`, which is really supposed to be just for dealing
with raw python-returned values, not `py::object`-derived wrappers which
are supposed to be higher level.

This changes the constructor of such converting types (i.e. anything
using PYBIND11_OBJECT_CVT -- `str`, `bool_`, `int_`, `float_`, `tuple`,
`dict`, `list`, `set`, `memoryview`) to reference rather than copy when
the check function passes.

It also adds an `object &&` constructor that is slightly more efficient
by avoiding an inc_ref when the check function passes.

tests/test_pytypes.py

@@ -174,9 +174,20 @@ def test_constructors():
     }
     inputs = {k.__name__: v for k, v in data.items()}
     expected = {k.__name__: k(v) for k, v in data.items()}
+
     assert m.converting_constructors(inputs) == expected
     assert m.cast_functions(inputs) == expected
 
+    # Converting constructors and cast functions should just reference rather
+    # than copy when no conversion is needed:
+    noconv1 = m.converting_constructors(expected)
+    for k in noconv1:
+        assert noconv1[k] is expected[k]
+
+    noconv2 = m.cast_functions(expected)
+    for k in noconv2:
+        assert noconv2[k] is expected[k]
+
 
 def test_implicit_casting():
     """Tests implicit casting when assigning or appending to dicts and lists."""