Implement WITH-REPL-EVAL-BOUNDARY and STACK-FRAME/REPL-EVAL-BOUNDARY?

Implement WITH-REPL-EVAL-BOUNDARY and STACK-FRAME/REPL-EVAL-BOUNDARY?
to mark the boundary between stack frames that are part of the REPL
and those that are part of the expression being evaluated by the REPL.
This marker frame is a "subproblem" frame because that is the easiest
way to make it visible to the debuggers, but operationally it is a
"reduction" frame.

The runtime system's debugger currently ignores these marker frames;
at some point it should be modified to do something with them.