Add an unsupervise builtin and function to std::task

Calling task::unsupervise de-parents the current task, with the result
that failures do not propogate up the task tree.

src/rt/rust_builtin.cpp

@@ -425,6 +425,11 @@ task_join(rust_task *task, rust_task *join_task) {
     }
 }
 
+extern "C" CDECL void
+task_unsupervise(rust_task *task) {
+    task->unsupervise();
+}
+
 /* Debug builtins for std.dbg. */
 
 static void