Get rid of if/else

Per request: http://issue.cc/r3/2077

Conflicts:
	src/boot/natives.r
	src/core/n-control.c

Author: BrianHawley

src/boot/errors.r

@@ -31,7 +31,6 @@ Note: [
 	no-load:            [{cannot load: } :arg1]
 	exited:             [{exit occurred}]
 	deprecated:         {deprecated function not allowed}
-	else-gone:          {ELSE is obsolete - use the EITHER function}
 ]
 
 Syntax: [

src/boot/natives.r

@@ -186,11 +186,10 @@ halt: native [
 ]
 
 if: native [
-	{If condition is TRUE, evaluates the block.}
+	{If TRUE condition, return arg; evaluate blocks by default.}
 	condition
-	then-block [block!]
-	/else "If not true, evaluate this block"
-	else-block [block!]
+	true-branch
+	/only "Return block arg instead of evaluating it."
 ]
 
 loop: native [
@@ -306,9 +305,10 @@ try: native [
 ]
 
 unless: native [
-	{Evaluates the block if condition is not TRUE.}
+	{If FALSE condition, return arg; evaluate blocks by default.}
 	condition
-	block [block!]
+	false-branch
+	/only "Return block arg instead of evaluating it."
 ]
 
 until: native [

src/core/n-control.c

@@ -597,17 +597,6 @@ enum {
 }
 
 
-/***********************************************************************
-**
-*/	REBNATIVE(else)
-/*
-***********************************************************************/
-{
-	Trap0(RE_ELSE_GONE);
-	DEAD_END;
-}
-
-
 /***********************************************************************
 **
 */	REBNATIVE(exit)
@@ -625,16 +614,13 @@ enum {
 /*
 ***********************************************************************/
 {
-	REBVAL *cond = D_ARG(1);
-	REBVAL *body = D_ARG(2);
-
-	if (!D_REF(3)) {	// no /else
-		if (IS_FALSE(cond)) return R_NONE;
-	} else
-		if (IS_FALSE(cond)) body = D_ARG(4);
-
-	DO_BLK(body);
-	return R_TOS1;
+	if (IS_FALSE(D_ARG(1))) return R_NONE;
+	if (IS_BLOCK(D_ARG(2)) && !D_REF(3) /* not using /ONLY */) {
+		DO_BLK(D_ARG(2));
+		return R_TOS1;
+	} else {
+		return R_ARG2;
+	}
 }