Clean up symbol management

Stop using that bitfield for everything, including what can be determined otherwise
It also makes it easier to have a sane state, since some bits were (supposedly)
mutually exclusive

test/asm/pc-bank.err

@@ -1,5 +1,5 @@
 ERROR: pc-bank.asm(2):
     Source address $2a00 not in $FF00 to $FFFF
 ERROR: pc-bank.asm(11):
-    @'s bank is not known yet
+    Current bank is not known yet
 error: Assembly aborted (2 errors)!

test/asm/pc-def.err

@@ -1,2 +1,3 @@
 ERROR: pc-def.asm(1):
-    '@' is not allowed as argument to the DEF function
+    Label "@" is not a valid argument to DEF
+error: Assembly aborted (1 errors)!

test/asm/pc-def.out

@@ -0,0 +1 @@
+defined