Allow various predicates

[some-mthfka]

Hi! I have implemented caching for the google-translate package using pcache. One problem I encountered is, 'equal was needed to use strings as keys, so I changed the pcache-repository class to accept :test and pass it to make-hash-table. It would be nice to see the changes in master.

But before that, there is a bug of a sort:

Create a repo:

(setf repo (pcache-repository "scratch" :test 'equal))
(pcache-put repo "who's" 1337)

After restarting emacs, loading the repo works as expected:

(setf repo (pcache-repository "scratch" :test 'equal))
(pcache-get repo "who's")

--> 1337

however the format in which the repo is saved has aquired the test variable:

;; Object scratch
;; EIEIO PERSISTENT OBJECT
(pcache-repository "scratch"
                   :file "scratch"
                   :object-name "scratch"
                   :version "26.1/0.4"
                   :test equal
                   :entries #s(hash-table size 65 test equal rehash-size 1.5 rehash-threshold 0.8125 data ("who's" (pcache-entry "pcache-entry" :timestamp 1536775676.2360787 :value 1337)))
                   :timestamp 1536775678.6913579)

Running eval-defun on it results in eval: Symbol’s value as variable is void: equal
Changing :test equal to :test 'equal fixes the problem however.

(Once again, all this happens when using eval-defun directly, pcache reloads the cache just fine.)

Do you think this might be a serious bug? Or maybe there is a way to save :test equal as :test 'equal?