Fix quadratic-time behaviour of vector extend

The "extend" method for vectors defined in stl_bind.h used `reserve` to
allocate space for the extra growth. While this can sometimes make a
constant-factor improvement in performance, it can also cause
construction of a vector by repeated extension to take quadratic rather
than linear time, as memory is reallocated in small increments rather
than on an exponential schedule. For example, this Python code would
take time proportional to the square of the trip count:

```python
a = VectorInt([1, 2, 3])
b = VectorInt()
for i in range(100000):
    b.extend(a)
```

This commit removes the `reserve` call. The alternative would be to try
to add some smarter heuristics, but the standard library may well have
its own heuristics (the iterators are random access iterators, so it can
easily determine the number of items being added) and trying to add more
heuristics on top of that seems like a bad idea.

include/pybind11/stl_bind.h

@@ -137,7 +137,6 @@ void vector_modifiers(enable_if_t<std::is_copy_constructible<typename Vector::va
 
     cl.def("extend",
        [](Vector &v, const Vector &src) {
-           v.reserve(v.size() + src.size());
            v.insert(v.end(), src.begin(), src.end());
        },
        arg("L"),