Use std::optional for element value+value is set

[robUx4]

As noted in #111 that could be a good candidate to use more standard types, now that we moved to C++17. It would also make it impossible to dissociate the value set from the value that is set or not.

However it may cause some code issues depending how people read/write the values. This is also trickier to EbmlBinary which doesn't hold a simple value or even EbmlMaster on which we don't really read/write values.