Decay ~end~ isotopes to ~null~ isotopes

This pushes forward an evaluator concept that frame execution
can fill slots with ~end~ on a first pass, and then turn them
into nulls when the frame actually executes.

(It's similar to how unsets are turned into nulls.)

src/core/evaluator/c-action.c

@@ -901,6 +901,7 @@ Bounce Action_Executor(Frame(*) f)
         if (Is_Word_Isotope_With_Id(ARG, SYM_END)) {
             if (NOT_PARAM_FLAG(PARAM, ENDABLE))
                 fail (Error_No_Arg(f->label, KEY_SYMBOL(KEY)));
+            Init_Nulled(ARG);  // more convenient, use ^META for nuance
             continue;
         }
 

tests/functions/function.test.reb

@@ -484,6 +484,6 @@
         (the '1020) = (foo 1020)
         nihil' = (foo comment "HI")
         null' = (foo any [1 > 2, 3 > 4])
-        '~end~ = meta (foo)
+        null? (foo)
     ]
 )

tests/functions/invisible.test.reb

@@ -200,10 +200,10 @@
 
     ~no-arg~ !! (right-normal ||)
     (null? do [right-normal* ||])
-    ('~end~ = ^ do [right-normal*])
+    (null? do [right-normal*])
 
     ~no-arg~ !! (|| left-normal)
-    ('~end~ = ^ do [|| left-normal*])
+    (null? ^ do [|| left-normal*])
     (null? do [left-normal*])
 
     ~no-arg~ !! (|| left-defer)
@@ -212,7 +212,7 @@
 
     ('|| = do [right-soft ||])
     ('|| = do [right-soft* ||])
-    ('~end~ = ^ do [right-soft*])
+    (null? ^ do [right-soft*])
 
     ; !!! This was legal at one point, but the special treatment of left
     ; quotes when there is nothing to their right means you now get errors.
@@ -224,7 +224,7 @@
 
     ('|| = do [right-hard ||])
     ('|| = do [right-hard* ||])
-    ('~end~ = ^ do [right-hard*])
+    (null? do [right-hard*])
 
     ; !!! See notes above.
     ;