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/tests/fail-methodmap-as-rvalue.sp

@@ -0,0 +1,17 @@
+native printnum(x);
+
+enum X {
+};
+
+methodmap X {
+	public X(int y) {
+		printnum(y);
+		return view_as<X>(0);
+	}
+};
+
+public main()
+{
+	X x = X(5);
+	return X;
+}