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Now the user can say 

> :break add 13

at the ghci prompt and the debugger will use the first top level module as the target for the breakpoint

Made a bit faster the test which gets done every time a running program hits a dynamic breakpoint. I moved the bounds checking inside a DEBUG pragma and replaced (IArray.!) for unsafeAt

A big motivation to start with it was getting several independently useful functions out of the Ghci monad and into the IO monad instead. Working in debugger integration for Emacs via the ghc-api is helping me to improve reusability..
      While I was there, I tried to make the code less tangled, easier to understand, switched from implicit Exceptions to explicit Eithers in the bkptTable code, etc.

This patch should provide more informative messages to the user 

Previously, when in a breakpoint, :quit was used to continue execution.
This is not the right thing to do, so this patch restores :quit to its
original meaning whether or not ghci is in an inferior session.

The continue behavior is now provided by ":breakpoint continue".
I added a synonim command in :continue because it is much shorter,
but this is optional

A module loaded under debugging mode but owning no breakpoints was erroneously identified by ghci as a non-under debugging module, producing a confusing error msg when the user tried to set a breakpoint

The fix inserts an empty list of sites in the module-sites dictionary used by the debugger

Specifically, this disables the special support in the RTS for looking up the datacon name corresponding to an address. 
Correspondingly, the debugging commads in GHCi will not be available, and neither will the '-fdebugging' flag

The :print, :sprint and :force commands for GHCi.
This set of commands allows inspection of heap structures of the bindings in the interactive environment.
This is useful to observe lazyness and specially to inspect things with undespecified polymorphic types, as happens often in breakpoints.

This patch adds dynamic breakpoints to GHCi


There is a new ':breakpoint' command to manage breakpoints.
GHCi simply uses the breakpoint api functions in ghc-api to install itself as a client.
The mechanism used by GHCi to keep track of enabled breakpoints is a simple table.

When a breakpoint is hit, a new interactive session is launched and the bindings in the breakpoint are injected. Some commands are disabled in this sub session