Add a type-safe C++ interface to a registry

Add a type-safe C++ interface to a registry

This changes DECLARE_REGISTRY to add a type-safe interface.  This
interface is a C++ class that handles the details of registering a
key, and provides various useful methods, including policy-based
cleanup.

gdb/ChangeLog
2019-05-08  Tom Tromey  <tom@tromey.com>

	* registry.h (DECLARE_REGISTRY): Define the _key class.