Breakpoint class.
Implements temporary breakpoints, ignore counts, disabling and
(re)-enabling, and conditionals.
Breakpoints are indexed by number through bpbynumber and by
the (file, line) tuple using bplist. The former points to a
single instance of class Breakpoint. The latter points to a
list of such instances since there may be more than one
breakpoint per line.
When creating a breakpoint, its associated filename should be
in canonical form. If funcname is defined, a breakpoint hit will be
counted when the first line of that function is executed. A
conditional breakpoint always counts a hit.