Make ifs and asserts tail-recursive

The local Value object prevented g++ from making a tail call.  Not
clear why.  In any case, not using a temporary makes g++ do the tail
call.

src/libexpr/eval.hh

@@ -145,6 +145,7 @@ public:
 
     /* Evaluation the expression, then verify that it has the expected
        type. */
+    inline bool evalBool(Env & env, Expr * e, Value & v);
     inline bool evalBool(Env & env, Expr * e);
     inline void evalAttrs(Env & env, Expr * e, Value & v);