Splitting out detail/type_caster_base.h from cast.h (NO other changes).

Author: rwgk

CMakeLists.txt

@@ -105,6 +105,7 @@ set(PYBIND11_HEADERS
     include/pybind11/detail/descr.h
     include/pybind11/detail/init.h
     include/pybind11/detail/internals.h
+    include/pybind11/detail/type_caster_base.h
     include/pybind11/detail/typeid.h
     include/pybind11/attr.h
     include/pybind11/buffer_info.h