Add AS and ALIASES? natives

Within the system there are ways to get the same series aliased as
different types.  Rather than the uphill battle of outlawing this, it
seems that the system benefits from being able to re-image a series
as another type.

src/core/m-gc.c

@@ -1351,7 +1351,6 @@ REBCNT Recycle_Core(REBOOL shutdown)
         // Mark value stack (temp-saved values):
         vp = SER_HEAD(REBVAL*, GC_Value_Guard);
         for (n = SER_LEN(GC_Value_Guard); n > 0; n--, vp++) {
-            if (NOT_END(*vp))
             if (NOT_END(*vp) && !IS_VOID_OR_SAFE_TRASH(*vp))
                 Queue_Mark_Value_Deep(*vp);
             Propagate_All_GC_Marks();

src/core/n-control.c

@@ -880,7 +880,6 @@ REBNATIVE(continue)
 //          binary! ;-- treated as UTF-8
 //          url! ;-- load code from URL via protocol
 //          file! ;-- load code from file on local disk
-//          group! ;-- !!! likely not needed with the AS aliasing
 //          tag! ;-- proposed as module library tag name, hacked as demo
 //          error! ;-- should use FAIL instead
 //          function! ;-- will only run arity 0 functions (avoids DO variadic)

src/core/n-data.c

@@ -1317,6 +1317,69 @@ REBNATIVE(punctuates_q)
 }
 
 
+//
+//  as: native [
+//
+//  {Aliases the underlying data of one series to act as another of same class}
+//
+//      type [datatype!]
+//      value [any-series!]
+//  ]
+//
+REBNATIVE(as)
+{
+    PARAM(1, type);
+    PARAM(2, value);
+
+    enum Reb_Kind kind = VAL_TYPE_KIND(ARG(type));
+    REBVAL *value = ARG(value);
+
+    switch (kind) {
+    case REB_BLOCK:
+    case REB_GROUP:
+    case REB_PATH:
+    case REB_LIT_PATH:
+    case REB_GET_PATH:
+        if (!ANY_ARRAY(value))
+            fail (Error_Invalid_Arg(value));
+        break;
+
+    case REB_STRING:
+    case REB_TAG:
+    case REB_FILE:
+    case REB_URL:
+        if (!ANY_BINSTR(value) || IS_BINARY(value))
+            fail (Error_Invalid_Arg(value));
+        break;
+    }
+
+    VAL_SET_TYPE_BITS(value, kind);
+    *D_OUT = *value;
+    return R_OUT;
+}
+
+
+//
+//  aliases?: native [
+//
+//  {Return whether or not the underlying data of one value aliases another}
+//
+//     value1 [any-series!]
+//     value2 [any-series!]
+//  ]
+//
+REBNATIVE(aliases_q)
+{
+    PARAM(1, value1);
+    PARAM(2, value2);
+
+    if (VAL_SERIES(ARG(value1)) == VAL_SERIES(ARG(value2)))
+        return R_TRUE;
+
+    return R_FALSE;
+}
+
+
 //
 //  set?: native [
 //