Adds the inspector commands to lispkit

Keybinds:

- `C-x-i-o`: opens inspector
- `C-x-i-c`: closes inspector
- `C-x-i-a`: attaches inspector
- `C-x-i-d`: detaches inspector

`C-x-i` was chosen because "inspector".
@Ralt sure, but I assume that setting the option is
idempotent and thus it is simpler to just set the value.
No biggie, though.