msgpack-rpc: Allow selective deferral API calls

Since all API functions now run immediately after a msgpack-rpc request is
parsed by libuv callbacks, a mechanism was added to override this behavior and
allow certain functions to run in Nvim main loop.

The mechanism is simple: Any API function tagged with the FUNC_ATTR_DEFERRED (a
"dummy" attribute only used by msgpack-gen.lua) will be called when Nvim main
loop receives a K_EVENT key.

To implement this mechanism it was necessary some restructuration on the
msgpack-rpc modules, especially in the msgpack-gen.lua script.

src/nvim/msgpack_rpc/defs.h

@@ -6,9 +6,15 @@
 
 /// The rpc_method_handlers table, used in msgpack_rpc_dispatch(), stores
 /// functions of this type.
-typedef Object (*rpc_method_handler_fn)(uint64_t channel_id,
-                                        msgpack_object *req,
-                                        Error *error);
+typedef struct {
+  Object (*fn)(uint64_t channel_id,
+               uint64_t request_id,
+               Array args,
+               Error *error);
+  bool defer;  // Should the call be deferred to the main loop? This should
+               // be true if the function mutates editor data structures such
+               // as buffers, windows, tabs, or if it executes vimscript code.
+} MsgpackRpcRequestHandler;
 
 /// Initializes the msgpack-rpc method table
 void msgpack_rpc_init_method_table(void);
@@ -31,4 +37,7 @@ Object msgpack_rpc_dispatch(uint64_t channel_id,
                             Error *error)
   FUNC_ATTR_NONNULL_ARG(2) FUNC_ATTR_NONNULL_ARG(3);
 
+MsgpackRpcRequestHandler msgpack_rpc_get_handler_for(const char *name,
+                                                     size_t name_len)
+  FUNC_ATTR_NONNULL_ARG(1);
 #endif  // NVIM_MSGPACK_RPC_DEFS_H