Clean up symbol system

Get rid of Hungarian notation
Improve encapsulation (the rest of the world should not touch PC directly)

test/asm/macro-@.err

@@ -1,4 +1,5 @@
 ERROR: [email protected](1):
     Label "foo" created outside of a SECTION
 ERROR: [email protected](1):
-    Macro '@' not defined
+    "@" is not a macro
+error: Assembly aborted (2 errors)!

test/asm/reference-undefined-sym.asm

@@ -1,4 +1,4 @@
-SECTION "sec", ROM0
+SECTION "sec", ROM0[0]
 	db X
 
 X = 2

test/asm/reference-undefined-sym.err

@@ -1,3 +0,0 @@
-ERROR: reference-undefined-sym.asm(4):
-    'X' already referenced at reference-undefined-sym.asm(2)
-error: Assembly aborted (1 errors)!

test/asm/symbol-override.err

@@ -3,5 +3,5 @@ ERROR: symbol-override.asm(6):
 ERROR: symbol-override.asm(10):
     'X' already defined as constant at symbol-override.asm(9)
 ERROR: symbol-override.asm(14):
-    'Y' already defined as non-constant at symbol-override.asm(13)
+    'Y' already defined as label at symbol-override.asm(13)
 error: Assembly aborted (3 errors)!