Add support for xref

[lokedhs]

I don't know much about how xref providers are implemented, but the starting point is the variable xref-backend-functions which should return an xref backend that is able to find symbols in the current runtime.

[larsbrinkhoff]

Source code is documentation:
https://github.com/emacs-mirror/emacs/blob/master/lisp/progmodes/xref.el

[phikal]

A basic Xref backend could be written that manually parses buffer for definitions, but are there any more generic tools (aside from Etags) that could provide the necessary information?

[larsbrinkhoff]

One idea I had when I started this project is that the Lisp code should be able to communicate with the inferior Forth for information. E.g. I seem to recall some Forths have built-in tools for cross referencing.