Make `overload_cast_impl` available in C++11 mode. (#1581)

* Make `overload_cast_impl` available in C++11 mode.

Narrow the scope of the `#if defined(PYBIND11_CPP14)` block around overload_cast to only
cover the parts where C++14 is stricly required. Thus, the implementation in
`pybind11::details::overload_cast_impl` is still available in C++11 mode.

* PR #1581: Modify test to use overload_cast_impl, update docs and change log