This commit improves the handling of hardcoded breakpoints in Delve.
A hardcoded breakpoint is a breakpoint instruction hardcoded in the
text of the program, for example through runtime.Breakpoint.

1. hardcoded breakpoints are now indicated by setting the breakpoint
   field on any thread stopped by a hardcoded breakpoint
2. if multiple hardcoded breakpoints are hit during a single stop all
   will be notified to the user.
3. a debugger breakpoint with an unmet condition can't hide a hardcoded
   breakpoint anymore.