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/keystore.h

@@ -13,6 +13,7 @@
 #include <boost/variant.hpp>
 
 class CScript;
+class CScriptID;
 
 /** A virtual base class for key stores */
 class CKeyStore