Write Stacktrace class

[floitschG]

We need to specify the Stacktrace class.

[gbracha]

FWIW spec now describes the desired type of the stack trace. It is of static type Object, and supports a toString method that prints out the trace. Some requirements on the trace contents are mandated.

---

Added Accepted label.

[lrhn]

We now specify that stacktraces have the type StackTrace.

---

Added Fixed label.