remove '#include <iostream>' from headers and from unneeded places

It's harmful to have iostream everywhere as it injects functions in the compiled files

src/model/model_pp.h

@@ -19,7 +19,7 @@ Revision History:
 --*/
 #pragma once
 
-#include<iostream>
+#include<ostream>
 class model_core;
 
 void model_pp(std::ostream & out, model_core const & m);

src/model/model_v2_pp.cpp

@@ -18,6 +18,7 @@ Revision History:
 #include "model/model_v2_pp.h"
 #include "model/model_core.h"
 #include "ast/ast_pp.h"
+#include <iostream>
 
 static void display_function(std::ostream & out, model_core const & md, func_decl * f, bool partial) {
     ast_manager & m = md.get_manager();

src/model/model_v2_pp.h

@@ -17,7 +17,7 @@ Revision History:
 --*/
 #pragma once
 
-#include<iostream>
+#include<ostream>
 class model_core;
 
 void model_v2_pp(std::ostream & out, model_core const & m, bool partial = false);