Add lambda expressions with closures to Zeek.

This allows anonymous functions in Zeek to capture their closures.
they do so by creating a copy of their enclosing frame and joining
that with their own frame.

There is no way to specify what specific items to capture from the
closure like C++, nor is there a nonlocal keyword like Python.
Attemptying to declare a local variable that has already been caught
by the closure will error nicely. At the worst this is an inconvenience
for people who are using lambdas which use the same variable names
as their closures.

As a result of functions copying their enclosing frames there is no
way for a function with a closure to reach back up and modify the
state of the frame that it was created in. This lets functions that
generate functions work as expected. The function can reach back and
modify its copy of the frame that it is captured in though.

Implementation wise this is done by creating two new subclasses in
Zeek. The first is a LambdaExpression which can be thought of as a
function generator. It gathers all of the ingredients for a function
at parse time, and then when evaluated creats a new version of that
function with the frame it is being evaluated in as a closure. The
second subclass is a ClosureFrame. This acts for most intents and
purposes like a regular Frame, but it routes lookups of values to its
closure as needed.

src/Var.cc

@@ -385,6 +385,7 @@ void begin_func(ID* id, const char* module_name, function_flavor flavor,
 
 	RecordType* args = t->Args();
 	int num_args = args->NumFields();
+
 	for ( int i = 0; i < num_args; ++i )
 		{
 		TypeDecl* arg_i = args->FieldDecl(i);
@@ -421,6 +422,7 @@ TraversalCode OuterIDBindingFinder::PreExpr(const Expr* expr)
 
 	const NameExpr* e = static_cast<const NameExpr*>(expr);
 
+	// TODO: Do we need to capture these as well?
 	if ( e->Id()->IsGlobal() )
 		return TC_CONTINUE;
 
@@ -431,17 +433,11 @@ TraversalCode OuterIDBindingFinder::PreExpr(const Expr* expr)
 	return TC_CONTINUE;
 	}
 
-void end_func(Stmt* body)
+// Gets a function's priority from its Scope's attributes. Errors if it sees any
+// problems.
+int get_func_priotity(attr_list* attrs)
 	{
-	int frame_size = current_scope()->Length();
-	id_list* inits = current_scope()->GetInits();
-
-	Scope* scope = pop_scope();
-	ID* id = scope->ScopeID();
-
 	int priority = 0;
-	auto attrs = scope->Attrs();
-
 	if ( attrs )
 		{
 		loop_over_list(*attrs, i)
@@ -473,27 +469,63 @@ void end_func(Stmt* body)
 			priority = v->InternalInt();
 			}
 		}
+		return priority;
+	}
 
-	if ( streq(id->Name(), "anonymous-function") )
+void end_func(Stmt* body)
+	{
+	std::unique_ptr<function_ingredients> ingredients =
+		gather_function_ingredients(body);
+	pop_scope();
+
+	if ( streq(ingredients->id->Name(), "anonymous-function") )
 		{
-		OuterIDBindingFinder cb(scope);
-		body->Traverse(&cb);
+		OuterIDBindingFinder cb(ingredients->scope);
+		ingredients->body->Traverse(&cb);
 
 		for ( size_t i = 0; i < cb.outer_id_references.size(); ++i )
 			cb.outer_id_references[i]->Error(
 						"referencing outer function IDs not supported");
 		}
 
-	if ( id->HasVal() )
-		id->ID_Val()->AsFunc()->AddBody(body, inits, frame_size, priority);
+	if ( ingredients->id->HasVal() )
+		ingredients->id->ID_Val()->AsFunc()->AddBody(
+			ingredients->body,
+			ingredients->inits,
+			ingredients->frame_size,
+			ingredients->priority);
 	else
 		{
-		Func* f = new BroFunc(id, body, inits, frame_size, priority);
-		id->SetVal(new Val(f));
-		id->SetConst();
+		Func* f = new BroFunc(
+			ingredients->id,
+			ingredients->body,
+			ingredients->inits,
+			ingredients->frame_size,
+			ingredients->priority);
+		ingredients->id->SetVal(new Val(f));
+		ingredients->id->SetConst();
 		}
 
-	id->ID_Val()->AsFunc()->SetScope(scope);
+	ingredients->id->ID_Val()->AsFunc()->SetScope(ingredients->scope);
+	}
+
+// Gathers all of the information from the current scope needed to build a
+// function and collects it into a function_ingredients struct.
+std::unique_ptr<function_ingredients> gather_function_ingredients(Stmt* body)
+	{
+	std::unique_ptr<function_ingredients> ingredients (new function_ingredients);
+	ingredients->frame_size = current_scope()->Length();
+	ingredients->inits = current_scope()->GetInits();
+
+	ingredients->scope = current_scope();
+	ingredients->id = ingredients->scope->ScopeID();
+
+	auto attrs = ingredients->scope->Attrs();
+
+	ingredients->priority = get_func_priotity(attrs);
+	ingredients->body = body;
+
+	return std::move(ingredients);
 	}
 
 Val* internal_val(const char* name)
@@ -508,6 +540,19 @@ Val* internal_val(const char* name)
 	return rval;
 	}
 
+std::shared_ptr<id_list> gather_outer_ids(Scope* scope, Stmt* body)
+	{
+	OuterIDBindingFinder cb(scope);
+	body->Traverse(&cb);
+
+	std::shared_ptr<id_list> idl (new id_list);
+
+	for ( size_t i = 0; i < cb.outer_id_references.size(); ++i )
+		idl->append(cb.outer_id_references[i]->Id());
+
+	return std::move(idl);
+	}
+
 Val* internal_const_val(const char* name)
 	{
 	ID* id = lookup_ID(name, GLOBAL_MODULE_NAME);