include cleanup

The Zeek code base has very inconsistent #includes.  Many sources
included a few headers, and those headers included other headers, and
in the end, nearly everything is included everywhere, so missing
#includes were never noticed.  Another side effect was a lot of header
bloat which slows down the build.

First step to fix it: in each source file, its own header should be
included first to verify that each header's includes are correct, and
none is missing.

After adding the missing #includes, I replaced lots of #includes
inside headers with class forward declarations.  In most headers,
object pointers are never referenced, so declaring the function
prototypes with forward-declared classes is just fine.

This patch speeds up the build by 19%, because each compilation unit
gets smaller.  Here are the "time" numbers for a fresh build (with a
warm page cache but without ccache):

Before this patch:

 3144.94user 161.63system 3:02.87elapsed 1808%CPU (0avgtext+0avgdata 2168608maxresident)k
 760inputs+12008400outputs (1511major+57747204minor)pagefaults 0swaps

After this patch:

 2565.17user 141.83system 2:25.46elapsed 1860%CPU (0avgtext+0avgdata 1489076maxresident)k
 72576inputs+9130920outputs (1667major+49400430minor)pagefaults 0swaps

src/Func.h

@@ -2,16 +2,21 @@
 
 #pragma once
 
+#include "BroList.h"
+#include "Obj.h"
+#include "Type.h" /* for function_flavor */
+#include "TraverseTypes.h"
+
 #include <utility>
 #include <memory>
+#include <string>
+#include <vector>
 
 #include <broker/data.hh>
 #include <broker/expected.hh>
 
-#include "BroList.h"
-#include "Obj.h"
-#include "Debug.h"
-#include "Frame.h"
+using std::string;
+using std::vector;
 
 class Val;
 class ListExpr;
@@ -20,6 +25,7 @@ class Stmt;
 class Frame;
 class ID;
 class CallExpr;
+class Scope;
 
 class Func : public BroObj {
 public: