The 'stack' variable could be #f when code is interpreted, which in
practice happens when running in "legacy" mode--i.e., when
'open-inferior' invokes "guile" instead of "guix repl".
* guix/repl.scm (send-repl-response)[handle-exception]: Check whether
STACK is true before passing it to 'stack->frames'.
* tests/inferior.scm ("&inferior-exception, legacy mode"): New test.
* guix/repl.scm (repl-prompt): New variable.
(stack->frames): New procedure.
(send-repl-response)[frame->sexp, handle-exception]: New procedure.
Pass HANDLE-EXCEPTION as a pre-unwind handler.
(machine-repl): Define 'tag'. Bump protocol version to (0 1 1).
Wrap 'loop' call in 'call-with-prompt'.
* guix/repl.scm (send-repl-response): Add #:version.
(machine-repl): Make 'loop' an internal define with a
'version' parameter. Pass VERSION to 'send-repl-response'.
Send (0 1) as the protocol version.
If the first element read from INPUT matches (() repl-version _ ...),
interpret it as the client's protocol version.
