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/Debug.h

@@ -2,10 +2,15 @@
 
 #pragma once
 
+#include "Obj.h"
+#include "Queue.h"
+#include "StmtEnums.h"
+
 #include <vector>
 #include <map>
 #include <string>
 
+class Val;
 class Stmt;
 
 // This needs to be defined before we do the includes that come after it.
@@ -17,17 +22,10 @@ struct ParseLocationRec {
 	int line;
 };
 
-#include "Expr.h"
-#include "Var.h"
-#include "Frame.h"
-#include "Queue.h"
-#include "Dict.h"
-#include "StmtEnums.h"
-#include "DbgBreakpoint.h"
-
 class StmtLocMapping;
 typedef PQueue<StmtLocMapping> Filemap; // mapping for a single file
 
+class Frame;
 class DbgBreakpoint;
 class DbgWatch;
 class DbgDisplay;