Implement order-independent purge (#1173)

Each symbol passed to purge is collected in a list before mass removal.
Fixes the issue described in gbdev/rgbds issue #1152.

---------

Co-authored-by: Rangi42 <[email protected]>

test/asm/purge-deferred.asm

@@ -0,0 +1,11 @@
+DEF prefix EQUS "cool"
+DEF {prefix}banana EQU 1
+
+ASSERT DEF(prefix)
+ASSERT DEF(coolbanana)
+
+; purging `prefix` should not prevent expanding it to purge `coolbanana`
+PURGE prefix, {prefix}banana
+
+ASSERT !DEF(prefix)
+ASSERT !DEF(coolbanana)