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/rpcmisc.cpp

@@ -292,7 +292,7 @@ Value createmultisig(const Array& params, bool fHelp)
 
     // Construct using pay-to-script-hash:
     CScript inner = _createmultisig_redeemScript(params);
-    CScriptID innerID = inner.GetID();
+    CScriptID innerID(inner);
     CBitcoinAddress address(innerID);
 
     Object result;