Refactor to move _fixup_expr_for_eval() into the hdb App and out of the debugger core
Refactor how eval is called and used

Renamed the function to _eval_in_program_context() to better reflect what it
is doing.  It's now a normal function call instead of expecting args in
the $DB::eval_string variable.

Debugger clients request for an eval with the $db->eval() method.  All the
evals are done the next time through the client loop.  Results are delivered
by a callback.
Remove the old "long_call" functionality

The App can now use the new notify_stopped() method to report back
to the browser.