Removing use of $assignable and deprecate it. Fix regression for stacktraces on MacOS. Added readline_to_stream. Regression: Chaining an optional together with contracts could in some cases lose the optional.

lib/std/os/macos/darwin.c3

@@ -88,8 +88,8 @@ fn String? executable_path()
 		char[4096] buf;
 		uint temp_len = buf.len;
     	if (darwin_NSGetExecutablePath(&buf, &temp_len) < 0) return NOT_FOUND?;
-    	path[:temp_len] = buf[:temp_len];
-    	len = temp_len;
+    	path[:len] = buf[:len];
+    	len = (int)((ZString)&buf).len();
     }
     return (String)path[:len];
 }