Add "sym" and "def" instructions, add argument naming

examples/aliases.csn

@@ -0,0 +1,13 @@
+(
+    (def FOO 123) ; define a constant
+    (call my-add 7 8)
+    (cmp res0 138 (ne? (fault "assert failed")))
+    (halt)
+
+    (proc my-add a b ; give arguments names
+        (sym q r0) ; give register a temporary alias
+        (add q a b)
+        (add q FOO)
+        (ret q)
+    )
+)

examples/factorial.csn

@@ -1,9 +1,8 @@
 (
-    (proc main
-        (call fac 5)
-        (ld r0 res0)
-        (halt)
-    )
+    (call fac 5)
+    (ld r0 res0)
+    (halt)
+
     (proc fac
         (cmp arg0 2
             (eq? (ret 2)))

examples/screen_bounce.csn

@@ -1,26 +1,24 @@
 ; Set log level to "info" or above for the best results!
-
 (
-    (proc main
-        (sc-init 800 600)
-        (sc-opt 1 1) ; auto blit
-        (sc-opt 2 25) ; frame rate
+    (sc-init 800 600)
+    (sc-opt 1 1) ; auto blit
+    (sc-opt 2 25) ; frame rate
 
-        (ld r0 5) ; x
-        (ld r1 0) ; y
+    (ld r0 5) ; x
+    (ld r1 0) ; y
 
-        (ld r2 1) ; dx
-        (ld r3 1) ; dy
+    (ld r2 1) ; dx
+    (ld r3 1) ; dy
 
-        (ld r5 0x3300ff)
+    (ld r5 0x3300ff)
 
-        (:loop)
-            (add r5 0x000001)
-            (sc-px r0 r1 r5)
-            (add r0 r2)
-            (add r1 r3)
-            (cmp r0 799 (eq? (ld r2 -1)) (ne? (cmp r0 0 (eq? (ld r2 1)))))
-            (cmp r1 599 (eq? (ld r3 -1)) (ne? (cmp r1 0 (eq? (ld r3 1)))))
-        (j :loop)
-    )
+    (:loop)
+        (add r5 0x000001)
+        (sc-px r0 r1 r5)
+        (add r0 r2)
+        (add r1 r3)
+        (cmp r0 799 (eq? (ld r2 -1)) (ne? (cmp r0 0 (eq? (ld r2 1)))))
+        (cmp r1 599 (eq? (ld r3 -1)) (ne? (cmp r1 0 (eq? (ld r3 1)))))
+    (j :loop)
+    (fault "unreachable")
 )