Implement compound assignment operators for mutable constants

Fixes #943

test/asm/compound-assignment.asm

@@ -0,0 +1,40 @@
+macro try
+println \1, "\2:"
+def prefix equs \1
+{prefix}\2 = 10
+println \2 ; 10
+{prefix}\2 += 5
+println \2 ; 15
+{prefix}\2 -= 1
+println \2 ; 14
+{prefix}\2 *= 2
+println \2 ; 28
+{prefix}\2 /= 4
+println \2 ; 7
+{prefix}\2 %= 3
+println \2 ; 1
+{prefix}\2 |= 11
+println \2 ; 11
+{prefix}\2 ^= 12
+println \2 ; 7
+{prefix}\2 &= 21
+println \2 ; 5
+{prefix}\2 <<= 2
+println \2 ; 20
+{prefix}\2 >>= 1
+println \2 ; 10
+purge prefix
+endm
+
+  try "", p
+  try "def ", q
+  try "redef ", r
+
+_RS += 100
+println _RS
+
+__LINE__ *= 200
+println __LINE__
+
+UnDeFiNeD ^= 300
+println UnDeFiNeD