Add facade types to avoid using raw Broker types

By avoiding to use `broker::data` directly, we gain a degree of freedom
that allows us to swap out `broker::data` for something else (e.g.,
`broker::variant`) in the future. Furthermore, it also helps us to keep
Broker types "local" to the Broker manager and gives us a nicer
interface.

Also replaces uses of `broker::expected` with `std::optional`. While an
`expected `can carry additional information as to why a value is not
present, nothing in Zeek ever cared about that. Hence, using
`std::optional` removes an unnecessary dependency on a Broker detail
while also being more efficient (no extra heap allocation when no value
is present).

src/Frame.h

@@ -2,8 +2,6 @@
 
 #pragma once
 
-#include <broker/data.hh>
-#include <broker/expected.hh>
 #include <memory>
 #include <optional>
 #include <string>
@@ -21,6 +19,9 @@ namespace zeek {
 
 using ValPtr = IntrusivePtr<Val>;
 
+class BrokerListView;
+class BrokerData;
+
 namespace detail {
 
 class CallExpr;
@@ -171,7 +172,7 @@ public:
      * sequence of two-element vectors, the first element reflecting
      * the frame value, the second its type.
      */
-    broker::expected<broker::data> Serialize();
+    std::optional<BrokerData> Serialize();
 
     /**
      * Instantiates a Frame from a serialized one.
@@ -180,7 +181,7 @@ public:
      * and the second is the unserialized frame with reference count +1, or
      * null if the serialization wasn't successful.
      */
-    static std::pair<bool, FramePtr> Unserialize(const broker::vector& data);
+    static std::pair<bool, FramePtr> Unserialize(BrokerListView data);
 
     // If the frame is run in the context of a trigger condition evaluation,
     // the trigger needs to be registered.