Revamp how runtime errors and fiber switching is handled.

- Add Fiber.transferError(_).
- Primitives place runtime errors directly in the fiber instead of on
  the stack.
- Primitives that change fibers set it directly in the VM.
- Allow a fiber's error to be any object (except null).

doc/site/core/fiber.markdown

@@ -134,3 +134,7 @@ run. This returns `false` if the fiber is currently running or has yielded.
 ### **transfer**(value)
 
 **TODO**
+
+### **transferError**(error)
+
+**TODO**