libcc1: use variadic templates for callbacks

This patch completes the transition of libcc1 from the use of separate
template functions for different arities to the use of variadic
functions.  This is how I had wanted it to work from the very
beginning, and is possible now with C++11.

I had thought that variadic callbacks required C++17, but it turns out
that the approach taken here is basically equivalent to std::apply --
just a bit wordier.

libcc1

	* rpc.hh (argument_wrapper) <get>: Replace cast operator.
	(argument_wrapper<T *>) <get>: Likewise.
	(unmarshall): Add std::tuple overloads.
	(callback): Remove.
	(class invoker): New.
	* libcp1plugin.cc (plugin_init): Update.
	* libcp1.cc (libcp1::add_callbacks): Update.
	* libcc1plugin.cc (plugin_init): Update.
	* libcc1.cc (libcc1::add_callbacks): Update.
	* connection.cc (cc1_plugin::connection::do_wait): Update.