Add struct mg_queue and API, change examples/multi-threaded, add tests

src/queue.h

@@ -0,0 +1,38 @@
+#pragma once
+
+#include "arch.h"    // For size_t
+#include "config.h"  // For MG_ENABLE_ATOMIC
+
+#if MG_ENABLE_ATOMIC
+#include <stdatomic.h>
+#elif !defined(_Atomic)
+#define _Atomic
+#endif
+
+// Single producer, single consumer non-blocking queue
+//
+// Producer:
+//    void *buf;
+//    while (mg_queue_space(q, &buf) < len) WAIT();  // Wait for free space
+//    memcpy(buf, data, len);  // Copy data to the queue
+//    mg_queue_add(q, len);    // Advance q->head
+//
+// Consumer:
+//    void *buf;
+//    while ((len = mg_queue_next(q, &buf)) == MG_QUEUE_EMPTY) WAIT();
+//    mg_hexdump(buf, len);    // Handle message
+//    mg_queue_del(q);         // Delete message
+//
+struct mg_queue {
+  char *buf;
+  size_t len;
+  volatile _Atomic size_t tail;
+  volatile _Atomic size_t head;
+};
+
+#define MG_QUEUE_EMPTY ((size_t) ~0ul)  // Next message size when queue is empty
+
+void mg_queue_add(struct mg_queue *, size_t len);       // Advance head
+void mg_queue_del(struct mg_queue *);                   // Advance tail
+size_t mg_queue_space(struct mg_queue *, char **);      // Get free space
+size_t mg_queue_next(struct mg_queue *, char **);       // Get next message size