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/key.h

@@ -30,14 +30,6 @@ public:
     CKeyID(const uint160& in) : uint160(in) {}
 };
 
-/** A reference to a CScript: the Hash160 of its serialization (see script.h) */
-class CScriptID : public uint160
-{
-public:
-    CScriptID() : uint160(0) {}
-    CScriptID(const uint160& in) : uint160(in) {}
-};
-
 /** An encapsulated public key. */
 class CPubKey
 {