Add loop syntactic sugar; errors now use print instead of debug; add check for duplicate labels.

examples/loop.csn

@@ -0,0 +1,43 @@
+(
+    ; Unlabeled loop. Can be exited by jumping out
+
+    (loop
+        (nop)
+        (nop)
+        (nop)
+    )
+
+    (barrier)
+
+    ; The above is equivalent to:
+
+    (:label)
+    (nop)
+    (nop)
+    (nop)
+    (j :label)
+
+    (barrier)
+
+    ; The label name can be specified.
+    ; This adds a start label and ":label-end" at the end of the loop:
+
+    (loop :repeat
+        (nop)
+        (nop)
+        (j :repeat-end)
+        (nop)
+    )
+
+    (barrier)
+
+    ; The above is equivalent to: (labels changed to avoid a compile error)
+
+    (:repeat2)
+    (nop)
+    (nop)
+    (j :repeat2-end)
+    (nop)
+    (j :repeat2)
+    (:repeat2-end)
+)