Optimize and simplify haskell-ds-create-imenu-index with hashtable

The function has implemented a poor man's hash table by manually
enlisting keys and then using `cl-case` on values. Just replace this
with a hashtable and thus simplify the code considerably. This makes
it faster as well because the `cl-case + setq` is O(n) whereas
`puthash` is O(1).

The output of the function was tested on Pandoc's
`Writers/Powerpoint/Output.hs` file of 2841 lines and comparing the
hashsum of the output before the changes and after. It didn't change.

The commit is also basically a backport of a similar change on
purescript-mode that has been forked form haskell-mode at some point:
purescript-emacs/purescript-mode#27

Author: Hi-Angel

Diff for: haskell-decl-scan.el

@@ -537,11 +537,7 @@ datatypes) in a Haskell file for the `imenu' package."
   ;; These lists are nested using `(INDEX-TITLE . INDEX-ALIST)'.
   (let* ((bird-literate (haskell-ds-bird-p))
          (index-alist '())
-         (index-class-alist '()) ;; Classes
-         (index-var-alist '())   ;; Variables
-         (index-imp-alist '())   ;; Imports
-         (index-inst-alist '())  ;; Instances
-         (index-type-alist '())  ;; Datatypes
+         (imenu (make-hash-table :test 'equal))
          ;; The result we wish to return.
          result)
     (goto-char (point-min))
@@ -557,47 +553,23 @@ datatypes) in a Haskell file for the `imenu' package."
                  (posns (cdr name-posns))
                  (start-pos (car posns))
                  (type (cdr result)))
-                 ;; Place `(name . start-pos)' in the correct alist.
-                 (cl-case type
-                   (variable
-                    (setq index-var-alist
-                          (cl-acons name start-pos index-var-alist)))
-                   (datatype
-                    (setq index-type-alist
-                          (cl-acons name start-pos index-type-alist)))
-                   (class
-                    (setq index-class-alist
-                          (cl-acons name start-pos index-class-alist)))
-                   (import
-                    (setq index-imp-alist
-                          (cl-acons name start-pos index-imp-alist)))
-                   (instance
-                    (setq index-inst-alist
-                          (cl-acons name start-pos index-inst-alist)))))))
+            (puthash type
+                     (cons (cons name start-pos) (gethash type imenu '()))
+                     imenu))))
     ;; Now sort all the lists, label them, and place them in one list.
-    (when index-type-alist
-      (push (cons "Datatypes"
-                  (sort index-type-alist 'haskell-ds-imenu-label-cmp))
-            index-alist))
-    (when index-inst-alist
-      (push (cons "Instances"
-                  (sort index-inst-alist 'haskell-ds-imenu-label-cmp))
-            index-alist))
-    (when index-imp-alist
-      (push (cons "Imports"
-                  (sort index-imp-alist 'haskell-ds-imenu-label-cmp))
-            index-alist))
-    (when index-class-alist
-      (push (cons "Classes"
-                  (sort index-class-alist 'haskell-ds-imenu-label-cmp))
-            index-alist))
-    (when index-var-alist
+    (dolist (type '((datatype . "Datatypes") (instance . "Instances")
+                    (import   . "Imports")   (class    . "Classes")))
+      (when-let* ((curr-alist (gethash (car type) imenu)))
+        (push (cons (cdr type)
+                    (sort curr-alist 'haskell-ds-imenu-label-cmp))
+              index-alist)))
+    (when-let* ((var-alist (gethash 'variable imenu)))
       (if haskell-decl-scan-bindings-as-variables
           (push (cons "Variables"
-                      (sort index-var-alist 'haskell-ds-imenu-label-cmp))
+                      (sort var-alist 'haskell-ds-imenu-label-cmp))
                 index-alist)
         (setq index-alist (append index-alist
-                                  (sort index-var-alist 'haskell-ds-imenu-label-cmp)))))
+                                  (sort var-alist 'haskell-ds-imenu-label-cmp)))))
     ;; Return the alist.
     index-alist))