plugin: Add hook registration

I might have gone a bit overboard with the type-checking, but
typesafe_cb_cast is quite nice to use, so why not. The macro to
register a new hook encapsulates the entire flow from param
serialization, to dispatch, parsing and callback dispatch in one
bundle. I was tempted to have the callback outside of the
registration, but it's unlikely that we'll have two calls to the same
hook with different callbacks.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

lightningd/plugin.h

@@ -13,6 +13,11 @@
  */
 struct plugins;
 
+/**
+ * A plugin, exposed as a stub so we can pass it as an argument.
+ */
+struct plugin;
+
 /**
  * Create a new plugins context.
  */