Cleanup/improve PList usage and Event API

Majority of PLists are now created as automatic/stack objects,
rather than on heap and initialized either with the known-capacity
reserved upfront or directly from an initializer_list (so there's no
wasted slack in the memory that gets allocated for lists containing
a fixed/known number of elements).

Added versions of the ConnectionEvent/QueueEvent methods that take
a val_list by value.

Added a move ctor/assign-operator to Plists to allow passing them
around without having to copy the underlying array of pointers.

src/Var.cc

@@ -325,8 +325,7 @@ static void transfer_arg_defaults(RecordType* args, RecordType* recv)
 
 		if ( ! recv_i->attrs )
 			{
-			attr_list* a = new attr_list();
-			a->append(def);
+			attr_list* a = new attr_list{def};
 			recv_i->attrs = new Attributes(a, recv_i->type, true);
 			}