Rudimentary interactive debugger support

[talksmall]

It's been a while since I pushed any updates, but I have finally got
around to it!

The latest round of checkins incorporate a number of fixes to the
recompiler support, mainly relating to contexts and uncommon branch
deoptimization.

I have also enhanced the StackTraceInspector to incorporate
rudimentary debug support. Broadly summarized this amounts to the
addition of three buttons to each activation shown in the
StackTraceInspector labelled "Step into", "Step over" and "Step out"
and broadly defined as:

Step into - steps to the next steppable bytecode
Step over - steps over any intervening send until it returns to the
current activation
Step out - steps until it returns from the current activation

The relevant VM changes are available in r161 and there is a
compatible download in the Downloads section.

I plan to enhance this to allow inspection of the frame contents as
well as the in-place code editing expected in Smalltalk debuggers, but
I think this is a useful starting point and a more usable interface
than the command line evaluator.

Please give it a whirl and post any feedback you may have.

Regards, talksmall