Implemented length storage of allocated memory

lib/mem/alloc_windows.q

@@ -1,5 +1,12 @@
 import sys
 
 alloc(length Int) -> Pointer {
-	return sys.mmap(0, length, 0x0004, 0x3000)
+	x := sys.mmap(0, length, 0x0004, 0x3000)
+
+	if x <= 0 {
+		return x
+	}
+
+	store(x, 8, length)
+	return x + 8
 }