script: move CScriptID to standard.h and add a ctor for creating them from CScripts

This allows for a reversal of the current behavior.

This:
CScript foo;
CScriptID bar(foo.GetID());

Becomes:
CScript foo;
CScriptID bar(foo);

This way, CScript is no longer dependent on CScriptID or Hash();

src/script/compressor.h

@@ -8,6 +8,8 @@
 
 #include "script/script.h"
 
+class CScriptID;
+
 /** Compact serializer for scripts.
  *
  *  It detects common cases and encodes them much more efficiently.