don't enable mode by just loading library

Previously `elisp-slime-nave-mode' was enabled in
`ielm-mode' (explicitly) and `emacs-lisp-mode' (because the arguments
DOC and INIT-VALUE were switched in the mode definition.

Also add variable `elisp-slime-nav-mode-lighter'.
use sparse keymap, define it in one go
use cl-lib instead of cl
more conventional message
enforce use of spaces for intendation