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/Val.h

@@ -2,12 +2,6 @@
 
 #pragma once
 
-#include <vector>
-#include <list>
-#include <array>
-#include <unordered_map>
-
-#include "net_util.h"
 #include "Type.h"
 #include "Dict.h"
 #include "CompHash.h"
@@ -17,9 +11,18 @@
 #include "ID.h"
 #include "Scope.h"
 #include "Notifier.h"
-#include "IPAddr.h"
-#include "DebugLogger.h"
 #include "RE.h"
+#include "net_util.h"
+
+#include <vector>
+#include <list>
+#include <array>
+#include <unordered_map>
+
+#include <sys/types.h> // for u_char
+
+using std::vector;
+using std::string;
 
 // We have four different port name spaces: TCP, UDP, ICMP, and UNKNOWN.
 // We distinguish between them based on the bits specified in the *_PORT_MASK
@@ -50,6 +53,9 @@ class StringVal;
 class EnumVal;
 class OpaqueVal;
 
+class IPAddr;
+class IPPrefix;
+
 class StateAccess;
 
 class VectorVal;