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".
Why not enable developer extras unconditionally? Seems to me that's
what this code amounts to anyway.