Fix `char &` arguments being non-bindable

This changes the caster to return a reference to a (new) local `CharT`
type caster member so that binding lvalue-reference char arguments
works (currently it results in a compilation failure).

Fixes #1116
diff --git a/tests/test_builtin_casters.cpp b/tests/test_builtin_casters.cpp
index b73e96e..e5413c2 100644
--- a/tests/test_builtin_casters.cpp
+++ b/tests/test_builtin_casters.cpp
@@ -50,7 +50,9 @@
     // test_single_char_arguments
     m.attr("wchar_size") = py::cast(sizeof(wchar_t));
     m.def("ord_char", [](char c) -> int { return static_cast<unsigned char>(c); });
+    m.def("ord_char_lv", [](char &c) -> int { return static_cast<unsigned char>(c); });
     m.def("ord_char16", [](char16_t c) -> uint16_t { return c; });
+    m.def("ord_char16_lv", [](char16_t &c) -> uint16_t { return c; });
     m.def("ord_char32", [](char32_t c) -> uint32_t { return c; });
     m.def("ord_wchar", [](wchar_t c) -> int { return c; });