lang/crystal: crystal-mode & flycheck-crystal

+ Add flycheck integration for the Crystal language, powered by
  flycheck-crystal.
+ Use the ELPA source for crystal-mode. The former QUELPA soruce
  (dotmilk's fork) is no longer maintained.

modules/lang/crystal/config.el

@@ -9,3 +9,7 @@
           (:exec        . "%c %s")
           (:description . "Run Crystal script"))))
 
+
+(def-package! flycheck-crystal
+  :after crystal-mode
+  :config (add-hook 'crystal-mode-hook #'flycheck-mode))

modules/lang/crystal/packages.el

@@ -1,5 +1,6 @@
 ;; -*- no-byte-compile: t; -*-
 ;;; lang/crystal/packages.el
 
-(package! crystal-mode :recipe (:fetcher github :repo "dotmilk/emacs-crystal-mode"))
+(package! crystal-mode)
+(package! flycheck-crystal)