Add notion of "unbound" variability to the type system.

Now, when a type is declared without an explicit "uniform" or "varying"
qualifier, its variability is unbound; depending on the context of the
declaration, the variability is later finalized.

Currently, in almost all cases, types with unbound variability are
resolved to varying types; the one exception is typecasts like:
"(int)1"; in this case, the fact that (int) has unbound variability
carries through to the TypeCastExpr, which in turn notices that the
expression being type cast has uniform type and in turn will resolve
(int) to (uniform int).

Fixes issue #127.

decl.h

@@ -156,7 +156,7 @@ public:
     /** Returns the symbol associated with the declarator. */
     Symbol *GetSymbol() const;
 
-    void Print() const;
+    void Print(int indent) const;
 
     /** Position of the declarator in the source program. */
     const SourcePos pos;
@@ -199,7 +199,7 @@ public:
     Declaration(DeclSpecs *ds, std::vector<Declarator *> *dlist = NULL);
     Declaration(DeclSpecs *ds, Declarator *d);
 
-    void Print() const;
+    void Print(int indent) const;
 
     /** This method walks through all of the Declarators in a declaration
         and returns a fully-initialized Symbol and (possibly) and