Improve @bp

[toivoh]

Currently,

@bp x > 0

will expand into

x > 0 ? @bp : nothing

and the debugger will trap at both the if node and the @bp node. It would probably be better if the user would experience @bp x > 0 as an atomic breakpoint.

Related: @bp should probably change the debugger state directly, so that it can be used outside of instrumented code. Need to change test_macro_trap and test_modscope to use something else, and reserve @bp for the interactive debugger.