Merge remote-tracking branch 'origin/topic/timw/266-namespaces'

Merge adjustments:

- Preserved original `base_type_no_ref` argument type as ::TypeTag
- Removed superfluous #pragma guard around deprecated TableVal ctor
- Clarify NEWS regarding MetaHook{Pre,Post} deprecations
- Simplify some `::zeek::` qualifications to just `zeek::`
- Prefixed FORWARD_DECLARE_NAMESPACED macro with ZEEK_

* origin/topic/timw/266-namespaces:
  Disable some deprecation diagnostics for GCC
  Rename BroType to Type
  Update NEWS
  Review cleanup
  Move Type types to zeek namespace
  Move Flare/Pipe from the bro namespace to zeek::detail
  Move Attr to the zeek::detail namespace
  Move Trigger into the zeek::detail namespace
  Move ID to the zeek::detail namespace
  Move Anon.h into zeek::detail namespace
  Mark all of the aliased classes in plugin/Plugin.h deprecated, and fix all of the plugins that were using them
  Move all of the base plugin classes into the zeek::plugin namespace
  Expr: move all classes into zeek::detail
  Stmt: move Stmt classes into zeek::detail namespace
  Add utility macro for creating namespaced aliases for classes

src/util.h

@@ -61,6 +61,11 @@ extern HeapLeakChecker* heap_checker;
 #include <pthread_np.h>
 #endif
 
+// Note: macro for internal use only during deprecation/namespacing process.
+#define ZEEK_FORWARD_DECLARE_NAMESPACED(cls, ns) \
+	namespace ns { class cls; } \
+	using cls [[deprecated("Remove in v4.1. Use " #ns "::" #cls " instead.")]] = ns::cls;
+
 [[deprecated("Remove in v4.1. Use uint64_t instead.")]]
 typedef uint64_t uint64;
 [[deprecated("Remove in v4.1. Use uint32_t instead.")]]