That check doesn't make sense to me.

If there's a reason, @kevjames3 will probably notice and let me know... :)
Switch to how builtin debugger does this things

This speeds up the test suite and practically drops dependency on linecache but
it doesn't fix reload (yet) which was the original purpose...
Improve "edit" command:

* Changed default editor from "ex" to "vim".
* It accepts only a filename now.