Use static_cast instead of C-style casts for non-fundamental types

A C-style cast is equivalent to try casting in the following order:

1. const_cast(...)
2. static_cast(...)
3. const_cast(static_cast(...))
4. reinterpret_cast(...)
5. const_cast(reinterpret_cast(...))

By using static_cast<T>(...) explicitly we avoid the possibility
of an unintentional and dangerous reinterpret_cast. Furthermore
static_cast<T>(...) allows for easier grepping of casts.

src/script/script.h

@@ -567,7 +567,7 @@ public:
             pc += nSize;
         }
 
-        opcodeRet = (opcodetype)opcode;
+        opcodeRet = static_cast<opcodetype>(opcode);
         return true;
     }