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]>

include/asm/lexer.h

@@ -97,4 +97,11 @@ struct DsArgList {
 	struct Expression *args;
 };
 
+#define INITIAL_PURGE_ARG_SIZE 2
+struct PurgeArgList {
+	size_t nbArgs;
+	size_t capacity;
+	char **args;
+};
+
 #endif // RGBDS_ASM_LEXER_H