refactor(api): make freeing of return-value opt-in instead of opt out

As only a few API functions make use of explicit freeing of the return
value, make it opt-in instead. The arena is always present under the
hood, so `Arena *arena` arg now doesn't mean anything other than getting
access to this arena. Also it is in principle possible to return an
allocated value while still using the arena as scratch space for other
stuff (unlikely, but there no reason to not allow it).

src/nvim/api/private/dispatch.h

@@ -18,8 +18,8 @@ struct MsgpackRpcRequestHandler {
               ///< uv loop (the loop is run very frequently due to breakcheck).
               ///< If "fast" is false, the function is deferred, i e the call will
               ///< be put in the event queue, for safe handling later.
-  bool arena_return;  ///< return value is allocated in the arena (or statically)
-                      ///< and should not be freed as such.
+  bool ret_alloc;  ///< return value is allocated and should be freed using api_free_object
+                   ///< otherwise it uses arena and/or static memory
 };
 
 extern const MsgpackRpcRequestHandler method_handlers[];