Replace symbol proxies with type symbols.

Proxies were rather hacky and complicated, and only existed as a
workaround for oddities with constructors. This patch replaces them with
actual type symbols, a very tiny step to semantically getting rid of tags.

This greatly simplifies how we implement constructors, and paves the way
for using methodmap symbols in field expressions.

Since non-value symbols are new to spcomp1, we place a rather
pigeonholed check into primary() to make sure non-value symbols don't
escape into expressions.

sourcepawn/compiler/sc5-in.scp

@@ -156,7 +156,7 @@ static const char *errmsg[] = {
 /*110*/  "%s has already been defined (previously seen as %s)\n",
 /*111*/  "expected identifier - did you forget a type?\n",
 /*112*/  "constructor function must return tag %s\n",
-/*113*/  "cannot define constructor for \"%s\"; already exists as a %s\n",
+/*113*/  "constructor for \"%s\" already exists\n",
 /*114*/  "missing type, or %s must have the same name as %s \"%s\"\n",
 /*115*/  "cannot use delete, %s %s has no destructor\n",
 /*116*/  "no methodmap or class was found for %s\n",
@@ -217,6 +217,7 @@ static const char *errmsg[] = {
 /*171*/  "cannot use 'new' with non-object-like methodmap '%s'\n",
 /*172*/  "methodmap '%s' does not have a constructor\n",
 /*173*/  "'%s' is a newly reserved keyword that may be used in the future; use a different name as an identifier\n",
+/*174*/  "symbol '%s' is a type and cannot be used as a value\n",
 #else
   "\315e\306\227\266k\217:\235\277bu\201fo\220\204\223\012",
   "\202l\224\250s\205g\346\356e\233\201(\243\315\214\267\202) \253 f\255low ea\305 \042c\353e\042\012",