Move serialization to vector pattern into a base class function

[cryptocode]

Turns this recurring pattern:

rai::confirm_req message (block);
std::shared_ptr<std::vector<uint8_t>> bytes (new std::vector<uint8_t>);
{
	rai::vectorstream stream (*bytes);
	message.serialize (stream);
}

into:

rai::confirm_req message (block);
auto bytes = message.to_bytes ();

Sometimes to_bytes () can be used directly in parameter position, simplifying the code further.

To make this work in const contexts, serialize functions are now const