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/wallet.cpp

@@ -158,7 +158,7 @@ bool CWallet::LoadCScript(const CScript& redeemScript)
      * these. Do not add them to the wallet and warn. */
     if (redeemScript.size() > MAX_SCRIPT_ELEMENT_SIZE)
     {
-        std::string strAddr = CBitcoinAddress(redeemScript.GetID()).ToString();
+        std::string strAddr = CBitcoinAddress(CScriptID(redeemScript)).ToString();
         LogPrintf("%s: Warning: This wallet contains a redeemScript of size %i which exceeds maximum size %i thus can never be redeemed. Do not use address %s.\n",
             __func__, redeemScript.size(), MAX_SCRIPT_ELEMENT_SIZE, strAddr);
         return true;