Allow REDEF for EQU constants

Fixes #853

test/asm/def.asm

@@ -26,4 +26,5 @@ def constant equ 6*7 ; fails
 redef string equs "there"
 	println "{string}"
 
-redef constant equ 6*9 ; syntax error
+redef constant equ 6*9
+	println constant

test/asm/def.err

@@ -1,5 +1,3 @@
 ERROR: def.asm(23):
     'constant' already defined at def.asm(10)
-ERROR: def.asm(29):
-    syntax error, unexpected EQU, expecting SET or = or EQUS
-error: Assembly aborted (2 errors)!
+error: Assembly aborted (1 error)!

test/asm/def.out

@@ -7,3 +7,4 @@ here
 $0 $1 $5 $9
 $2A
 there
+$36

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

test/asm/redef-equ.err

@@ -0,0 +1,3 @@
+ERROR: redef-equ.asm(23):
+    'N' already defined as non-EQU at redef-equ.asm(22)
+error: Assembly aborted (1 error)!

test/asm/redef-equ.out

@@ -0,0 +1,2 @@
+$1
+$3$1$4$9

test/asm/reference-undefined-equs.asm

@@ -0,0 +1,5 @@
+SECTION "sec", ROM0[0]
+	db s1, s2
+
+def s1 equs "1"
+redef s2 equs "2"

test/asm/reference-undefined-equs.err

@@ -0,0 +1,5 @@
+ERROR: reference-undefined-equs.asm(4):
+    's1' already referenced at reference-undefined-equs.asm(2)
+ERROR: reference-undefined-equs.asm(5):
+    's2' already referenced at reference-undefined-equs.asm(2)
+error: Assembly aborted (2 errors)!

test/asm/reference-undefined-sym.asm

@@ -1,4 +1,7 @@
 SECTION "sec", ROM0[0]
-	db X
+	db x1, x2, y1, y2
 
-X = 2
+def x1 = 1
+redef x2 = 2
+def y1 equ 3
+redef y2 equ 4

test/asm/reference-undefined-sym.out.bin

@@ -0,0 +1 @@
+