rt::task: Make current_stack_segment public

Servo needs to tell SpiderMonkey about the stack bounds.

Author: Keegan McAllister

src/libstd/rt/task.rs

@@ -64,7 +64,7 @@ pub struct Coroutine {
     /// The segment of stack on which the task is currently running or
     /// if the task is blocked, on which the task will resume
     /// execution.
-    priv current_stack_segment: StackSegment,
+    current_stack_segment: StackSegment,
     /// Always valid if the task is alive and not running.
     saved_context: Context
 }