Stop exporting unsafe spawn functions

Issue #1022

Author: brson

src/lib/task.rs

@@ -14,9 +14,6 @@ export unsupervise;
 export pin;
 export unpin;
 export set_min_stack;
-export spawn;
-export spawn_notify;
-export spawn_joinable;
 export task_result;
 export tr_success;
 export tr_failure;