Rename Devel::hdb::Evalable -> Actionable
Refactor so subclasses more directly specify their type
When fabbing up the breakpoint, use a string instead of a subref

When sending the response, the JSON encoder barfed on the subref
Refactor so Breakpoints and Actions use common code on the Perl side
If the postponed file is already loaded, run the sub immediately
WIP refactor to move stack() into the debugger package
Fixed the way stack frames are reported

The previous code was sloppy about properties other than the filename,
line and subroutine.

The new code basically uses the package, file and line from the
prior caller in constructing the current stack frame.