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/primitives/block.h

@@ -86,14 +86,14 @@ public:
     CBlock(const CBlockHeader &header)
     {
         SetNull();
-        *((CBlockHeader*)this) = header;
+        *(static_cast<CBlockHeader*>(this)) = header;
     }
 
     ADD_SERIALIZE_METHODS;
 
     template <typename Stream, typename Operation>
     inline void SerializationOp(Stream& s, Operation ser_action) {
-        READWRITE(*(CBlockHeader*)this);
+        READWRITE(*static_cast<CBlockHeader*>(this));
         READWRITE(vtx);
     }