Make TypeErrors more informative when an optional header is missing

E.g. trying to convert a `list` to a `std::vector<int>` without
including <pybind11/stl.h> will now raise an error with a note that
suggests checking the headers.

The note is only appended if `std::` is found in the function
signature. This should only be the case when a header is missing.
E.g. when stl.h is included, the signature would contain `List[int]`
instead of `std::vector<int>` while using stl_bind.h would produce
something like `MyVector`. Similarly for `std::map`/`Dict`, `complex`,
`std::function`/`Callable`, etc.

There's a possibility for false positives, but it's pretty low.

tests/pybind11_cross_module_tests.cpp

@@ -114,4 +114,10 @@ PYBIND11_MODULE(pybind11_cross_module_tests, m) {
     // the same module (it would be an ODR violation). Therefore `bind_vector` of `bool`
     // is defined here and tested in `test_stl_binders.py`.
     py::bind_vector<std::vector<bool>>(m, "VectorBool");
+
+    // test_missing_header_message
+    // The main module already includes stl.h, but we need to test the error message
+    // which appears when this header is missing.
+    m.def("missing_header_arg", [](std::vector<float>) { });
+    m.def("missing_header_return", []() { return std::vector<float>(); });
 }