Register new snapshots, purge log_err and log_full in favour of log(...).

src/libcore/task.rs

@@ -22,7 +22,7 @@ Tasks may execute in parallel and are scheduled automatically by the runtime.
 Example:
 
 > spawn("Hello, World", fn (&&msg: str) {
->   log_full(core::debug, msg);
+>   log(debug, msg);
 > });
 
 */