Various cleanups and improvements to the breakpoint support

Various cleanups and improvements to the breakpoint support

  - move parts of the debugger implementation below the GHC API where
    they belong.  There is still more in Debugger that violates the
    layering, hopefully I'll get to that later.

  - instead of returning an IO action from runStmt for resuming,
    return a ResumeHandle that is passed to GHC.resume.

  - breakpoints now return [Name] which is displayed in the same
    way as when a binding statement is executed.

  - :load, :add, :reload now clear the active breakpoints and context

  - :break gives a sensible error when used on a non-interpreted module

  - export breakpoint-related types from GHC

  - remove a bunch of layer-violating imports from InteractiveUI

  - remove some more vestiges of the old breakpoint code (topLevel in
    the GHCi state).

  - remove TickTree and use a simple array instead, cached per module