Summary:
Breakpoints become SCCs, so we have detailed call-stack info for
interpreted code.  Currently this only works when GHC is compiled with
-prof, but D1562 (Remote GHCi) removes this constraint so that in the
future call stacks will be available without building your own GHCi.

How can you get a stack trace?

* programmatically: GHC.Stack.currentCallStack
* I've added an experimental :where command that shows the stack when
  stopped at a breakpoint
* `error` attaches a call stack automatically, although since calls to
  `error` are often lifted out to the top level, this is less useful
  than it might be (ImplicitParams still works though).
* Later we might attach call stacks to all exceptions

Other related changes in this diff:

* I reduced the number of places that get ticks attached for
  breakpoints.  In particular there was a breakpoint around the whole
  declaration, which was often redundant because it bound no variables.
  This reduces clutter...