Support `evil-undo-system` other than undo-tree

[fnussbaum]

As of https://github.com/emacs-evil/evil/commit/4a336e9c0e0940ad21716260be9bb393c89f6553, evil supports undo-fu and the built-in undo-only/undo-redo commands. The commands evil-undo and evil-redo will call the correct functions depending on the value of evil-undo-system.

Spacemacs switched to undo-fu by default in https://github.com/syl20bnr/spacemacs/commit/c8da713c6986e2c9236cb3d520a75beb0b64fb9d.