Breakpointing Debugger

A new general debugger is introduced, with advanced debugging features and an advice facility. It generalizes the notion of spypoint to that of the breakpoint. Breakpoints make it possible to e.g. stop the program at a specified line, or in a specified line range, or to call arbitrary Prolog goals at specified ports, etc. Highlights:

• Advice facility -- useful for non-interactive debugging, such as checking of program invariants, collecting information, profiling, etc.
• Debugger hook predicate -- new interactive tracer commands can be defined.
• Tracer information access -- data on current and past execution states, such as those contained in the ancestor list, or the backtrace, is now accessible to the program.
• The following built-in predicates have been added: add_breakpoint/2, spy/2, current_breakpoint/4, remove_breakpoints/1, disable_breakpoints/1, enable_breakpoints/1, execution_state/1, and execution_state/2. user:debugger_command_hook/2 is a new hook predicate.

  The predicates nospy/1 and nospyall/0 have slighty changed meaning.

  The predicate spypoint_condition/3 has been removed.