See [lix-project/lix#1156], but basically currently the `:st <n>`
debugger command doesn't allow any negative indices, and putting a plus
sign in front of the arg doesn't change anything; thus, we can exploit
that "design space" to allow users to move between different stack
frames easily, by simply prepending their arg with a +/- sign.
The actual behavior is little more nuanced when you account for errors:
as suggested by @pennae (thanks! :), when the user inputs an offset that
would result in an invalid frame index, the debugger instead clamps it
to the closest bound (i.e. 0 for negative offsets, $maxFrame for
positive ones) and just prints a warning.
Fixes#1156
[lix-project/lix#1156]: https://git.lix.systems/lix-project/lix/issues/1156
Change-Id: I02a0cdb6aaebbdb0515308880a3bf9c0d2fcd25e
