port: add $call atom to message tuple, and use gen_server like format

Change message format from `{Pid, MonitorRef, Message}` to
`{'$call', {self(), MonitorRef}, Message}`, for consistency and
future-proofing.

Signed-off-by: Davide Bettio <davide@uninstall.it>

libs/eavmlib/src/port.erl

@@ -68,7 +68,7 @@ call(Port, Message) ->
 -spec call(pid(), Message :: term(), Timeout :: timeout()) -> term() | {error, timeout}.
 call(Port, Message, Timeout) ->
     MonitorRef = monitor(port, Port),
-    Port ! {self(), MonitorRef, Message},
+    Port ! {'$call', {self(), MonitorRef}, Message},
     Result =
         receive
             {'DOWN', MonitorRef, port, Port, normal} ->

src/libAtomVM/port.c

@@ -111,17 +111,27 @@ enum GenMessageParseResult port_parse_gen_message(term msg, GenMessage *gen_mess
         return GenMessageParseError;
     }
 
-    gen_message->pid = term_get_tuple_element(msg, 0);
-    if (UNLIKELY(!term_is_pid(gen_message->pid))) {
+    gen_message->message_type = term_get_tuple_element(msg, 0);
+    if (UNLIKELY(!term_is_atom(gen_message->message_type))) {
         return GenMessageParseError;
     }
 
-    gen_message->ref = term_get_tuple_element(msg, 1);
-    if (UNLIKELY(!term_is_reference(gen_message->ref))) {
+    gen_message->from = term_get_tuple_element(msg, 1);
+    if (UNLIKELY(
+            !term_is_tuple(gen_message->from) || term_get_tuple_arity(gen_message->from) != 2)) {
         return GenMessageParseError;
     }
 
     gen_message->req = term_get_tuple_element(msg, 2);
 
+    gen_message->pid = term_get_tuple_element(gen_message->from, 0);
+    if (UNLIKELY(!term_is_pid(gen_message->pid))) {
+        return GenMessageParseError;
+    }
+    gen_message->ref = term_get_tuple_element(gen_message->from, 1);
+    if (UNLIKELY(!term_is_reference(gen_message->ref))) {
+        return GenMessageParseError;
+    }
+
     return GenMessageParseOk;
 }

src/libAtomVM/port.h

@@ -84,6 +84,7 @@ static inline void port_send_reply(Context *ctx, term pid, term ref, term payloa
 
 typedef struct
 {
+    term message_type;
     term from;
     term req;