Add checkpoint push/pop command

checkpoint push - Push an anonymous checkpoint and add it to a checkpoint stack
checkpoint pop - Restore to the current top of the checkpoint stack and pop
info checkpoint stack - (optional) List the current checkpoint stack

Having the above command would make it easier to build user define commands that need change the program state. For instance I'd like to have something like to make it easier to debug reftest:

define show frame tree
  checkpoint push
  <goto prev refresh driver tick>
  print "Current:"
  call frame->DumpFrameTree()
  <goto next refresh driver tick>
  print "Next:"
  call frame->DumpFrameTree()
  checkpoint pop
end

If we think it's a good idea I can build a PR for this.

Start by locating rr’s debugger command-handling entry points and the code that manages program-state checkpoints; the issue names no files or tests. Trace how user-defined commands invoke debugger commands, then add coverage for push/pop restoration and the optional stack listing. Done means the requested commands work together without leaving program state changed.