Closes #6: SPC like TAB when reading symbols from minibuffer

* sly.el (sly-read-symbol-name): Use `sly-minibuffer-read-symbol-map'
(sly-minibuffer-read-symbol-map): New keymap.
(sly-read-from-minibuffer): Take keymap arg.

* contrib/sly-fuzzy.el (sly-fuzzy-target-buffer-completions-mode): Be wary of buffer kills.