Add a new module implementing a queue data structure.

Since the functions are simple make them inline functions. This provides a
small code size saving.

tests/app_queue_test/src/app_queue_test.xc

@@ -0,0 +1,32 @@
+#include "queue.h"
+#include <stdlib.h>
+
+#define VERIFY(x) do { if (!(x)) _Exit(1); } while(0)
+
+#define ARRAY_SIZE(x) (sizeof(x) / sizeof(x[0]))
+
+int main()
+{
+    queue_t q;
+    unsigned array[4];
+    const unsigned size = ARRAY_SIZE(array);
+    queue_init(q, size);
+    VERIFY(queue_is_empty(q));
+    VERIFY(!queue_is_full(q));
+    VERIFY(queue_items(q) == 0);
+    VERIFY(queue_space(q) == size);
+    queue_push_word(q, array, 1);
+    VERIFY(!queue_is_empty(q));
+    VERIFY(queue_items(q) == 1);
+    VERIFY(queue_space(q) == size - 1);
+    for (unsigned i = 1; i < size; i++) {
+        queue_push_word(q, array, i + 1);
+    }
+    VERIFY(queue_is_full(q));
+    VERIFY(queue_items(q) == size);
+    VERIFY(queue_space(q) == 0);
+    for (unsigned i = 0; i < size; i++) {
+        VERIFY(queue_pop_word(q, array) == i + 1);
+    }
+    return 0;
+}