Add a vtable-based breakpoint ops

Add a vtable-based breakpoint ops

This adds methods to struct breakpoint.  Each method has a similar
signature to a corresponding function in breakpoint_ops, with the
exceptions of create_sals_from_location and create_breakpoints_sal,
which can't be virtual methods on breakpoint -- they are only used
during the construction of breakpoints.

Then, this adds a new vtable_breakpoint_ops structure and populates it
with functions that simply forward a call from breakpoint_ops to the
corresponding virtual method.  These are all done with lambdas,
because they are just a stepping stone -- by the end of the series,
this structure will be deleted.