msgpack-rpc: Create subdirectory for msgpack-rpc modules

Create the msgpack_rpc subdirectory and move all modules that deal with
msgpack-rpc to it. Also merge msgpack_rpc.c into msgpack_rpc/helpers.c

src/nvim/msgpack_rpc/defs.h

@@ -0,0 +1,34 @@
+#ifndef NVIM_MSGPACK_RPC_DEFS_H
+#define NVIM_MSGPACK_RPC_DEFS_H
+
+#include <msgpack.h>
+
+
+/// 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);
+
+/// Initializes the msgpack-rpc method table
+void msgpack_rpc_init_method_table(void);
+
+void msgpack_rpc_init_function_metadata(Dictionary *metadata);
+
+/// Dispatches to the actual API function after basic payload validation by
+/// `msgpack_rpc_call`. It is responsible for validating/converting arguments
+/// to C types, and converting the return value back to msgpack types.
+/// The implementation is generated at compile time with metadata extracted
+/// from the api/*.h headers,
+///
+/// @param channel_id The channel id
+/// @param method_id The method id
+/// @param req The parsed request object
+/// @param error Pointer to error structure
+/// @return Some object
+Object msgpack_rpc_dispatch(uint64_t channel_id,
+                            msgpack_object *req,
+                            Error *error)
+  FUNC_ATTR_NONNULL_ARG(2) FUNC_ATTR_NONNULL_ARG(3);
+
+#endif  // NVIM_MSGPACK_RPC_DEFS_H