Fix bug in dystring-append

If appending an empty string ("") the null terminator wouldn't be
removed, causing it to appear broken. Now, that condition is checked
for. I also made it get the length, then resize and strcpy, because I
think that will be faster anyways, especially on large strings.

src/DynamicArray.cake

@@ -163,16 +163,14 @@
   (assert (!= (deref str) append-str))
 
   ;; Remove the null terminator
-  (when (> (dynstring-strlen (deref str)) 0)
+  (when (or (> (dynstring-strlen (deref str)) 0)
+            (= 1 (dynarray-length (deref str)))) ;; Empty string
     (dynarray-pop (deref str)))
 
-  ;; using strcat may be faster if it is optimized. I won't worry about it for now
-  (each-char-in-string-const append-str current-char
-    ;; Will resize if necessary
-    (dynarray-push (deref str) (deref current-char)))
+  (var length-before-append size_t (dynarray-length (deref str)))
+  (dynarray-set-length (deref str) (+ length-before-append (strlen append-str) 1))
+  (strcpy (+ (deref str) length-before-append) append-str)
 
-  ;; Add the null terminator
-  (dynarray-push (deref str) 0)
   (return (dynstring-strlen (deref str))))
 
 ;; dynarray-length includes the null terminator. This function removes it