Rewrite buttercup-expect with pcase

Replace the special case of (null matcher) with using the
:to-be-truthy matcher.  buttercup-expect is probably never called
without a matcher as the expect macro will pass the :to-be-truthy
matcher if none is given.

buttercup.el

@@ -219,24 +219,14 @@ the arguments is delayed until the containing spec is executed."
 See the macro documentation for details and the definition of
 ARG, MATCHER and ARGS."
   (cl-assert (cl-every #'buttercup--wrapper-fun-p (cons arg args)) t)
-  (if (not matcher)
-      (progn
-        (cl-assert (not args) t)
-        (when (not (funcall arg))
-          (buttercup-fail "Expected %S to be non-nil"
-                          (buttercup--enclosed-expr arg))))
-    (let ((result (buttercup--apply-matcher matcher (cons arg args))))
-      (if (consp result)
-          (when (not (car result))
-            (buttercup-fail "%s" (cdr result)))
-        (when (not result)
-          (buttercup-fail "Expected %S %S %s"
-                          (buttercup--enclosed-expr arg)
-                          matcher
-                          (mapconcat (lambda (obj)
-                                       (format "%S" (funcall obj)))
-                                     args
-                                     " ")))))))
+  (pcase (buttercup--apply-matcher (or matcher :to-be-truthy) (cons arg args))
+    (`(,result . ,message) (unless result (buttercup-fail message)))
+    (`,result (unless result (buttercup-fail "Expected %S %S %s"
+                                            (buttercup--enclosed-expr arg)
+                                            matcher
+                                            (mapconcat (lambda (obj)
+                                                         (format "%S" (funcall obj)))
+                                                       args " "))))))
 
 (defun buttercup-fail (format &rest args)
   "Fail the current test with the given description.