Correctly insert the user's chosen completion candidate when
completing a prefix with slime-complete-symbol*.

For example, if the user types "cl:f-o" and then calls
slime-complete-symbol*, the *Completions* buffer will pop up and
display a list of possible completions. If the user then selects
"cl:force-output" from the *Completions* list, the resulting completed
symbol in the original buffer should be "cl:force-output". Prior to
this change, the resulting completion would be either
"cl:fcl:force-output-o" or "cl:f-ocl:force-output", depending on the
value of slime-c-p-c-unambiguous-prefix-p.

The fix entails setting the variable completion-base-position to a
list of (START END) markers indicating the portion of the target
buffer that should be replaced by choose-completion.

Prior to this change, completion-base-position was incorrectly set to
a single number, similar to what would have been expected by the
now-obsolete completion-base-size. This ultimately resulted in
completion-base-position being ignored in choose-completion-string:

  ;; Some older code may call us passing `base-size' instead of
  ;; `base-position'.  It's difficult to make any use of `base-size',
  ;; so we just ignore it.
  (unless (consp base-position)
    (message "Obsolete `base-size' passed to choose-completion-string")
    (setq base-position nil))

Note that completion-base-position was added (and completion-base-size
made obsolete) in emacs 23.2. Thus, completion-base-position is
ignored on emacs 23.1, but this was already the case prior to this
change. For emacs 23.1, if completion-base-size is unset, then
choose-completion-string will attempt to guess the region to replace
by calling choose-completion-delete-max-match.