Allow REDEF for EQU constants

Fixes #853

test/asm/redef-equ.asm

@@ -0,0 +1,23 @@
+DEF n EQU 0
+REDEF n EQU 1
+; prints "$1"
+PRINTLN n
+
+list: MACRO
+LIST_NAME EQUS "\1"
+DEF LENGTH_{LIST_NAME} EQU 0
+ENDM
+
+item: MACRO
+REDEF LENGTH_{LIST_NAME} EQU LENGTH_{LIST_NAME} + 1
+DEF {LIST_NAME}_{d:LENGTH_{LIST_NAME}} EQU \1
+ENDM
+
+	list SQUARES
+	item 1
+	item 4
+	item 9
+	println LENGTH_SQUARES, SQUARES_1, SQUARES_2, SQUARES_3
+
+N EQUS "X"
+REDEF N EQU 42