(read_minibuf): Use empty_string.

(read_minibuf): Use empty_string.
(Ftry_completion): Allow lambda forms and lists of strings for `alist'.
Short-circuit the search as soon as it "failed".
(Fall_completions): Allow lambda forms and lists of strings for alist.
(Fcompleting_read): Set Qminibuffer_completion_confirm to nil
when require_match is nil.
(Ftest_completion): Rename from `test_completion' and export to elisp.
Call the predicate also when alist is a list.
Obey Vcompletion_regexp_list.
(do_completion, Fminibuffer_complete_and_exit): Use it.
(Fassoc_string): Rename from `assoc_for_completion'.
Allow list of strings as well and export to elisp.