Robustly disallow overwriting builtin symbols (#817)

test/asm/builtin-overwrite.asm

@@ -0,0 +1,43 @@
+macro tickle
+; There once was a bug where overwriting worked only on the second try, so
+; try everything twice for good measure
+
+; Skip this syntax for EQUS, as it is invalid
+IF \2
+\1 = 0
+\1 = 0
+PRINTLN \1
+
+\1 EQU 0
+\1 EQU 0
+PRINTLN \1
+ENDC
+
+PURGE \1
+PURGE \1
+PRINTLN \1
+
+DEF \1 EQU 0
+DEF \1 EQU 0
+PRINTLN \1
+
+DEF \1 = 0
+DEF \1 = 0
+PRINTLN \1
+
+DEF \1 EQUS "hello"
+DEF \1 EQUS "hello"
+PRINTLN \1
+
+REDEF \1 = 0
+REDEF \1 = 0
+PRINTLN \1
+
+REDEF \1 EQUS "hello"
+REDEF \1 EQUS "hello"
+PRINTLN \1
+endm
+
+    ; Representative numeric and string builtins
+    tickle __LINE__, 1
+    tickle __FILE__, 0