Fixes problems with ReplRestart

[frere-jacques]

If one opens a REPL and then Restarts a REPL, one gets errors about wrong buffer ids.

The reason is that core.repl_restart deletes the old buffer. But the default of close_window_on_exit, will try to delete it too.

Also if one calls IronRestart from a code buffer, changes are done to meta, but new_meta is returned.

I made a commit that fixes this, by not altering meta, but altering new_meta.
Also it will be checked in the on exit option, whether the buffer still exist.

I will create a merge request.

I also observed that many functions will differ strongly whether the cursor is on code buffer or repl buffer. One could change that if the get_ft would check on empty file type, if the buffer is the buffer of a repl and then use the ft from store.
I checked that, but I am unsure whether this could have some side effects.

See #387 (comment)