Usability with msg::disjoint_field

[herculanodavi]

The msg::disjoint_field is very useful to me, but I'm going through the problems below and would be happy to make a PR with these changes:

Add access to the DWordIndex of the contained types

When making a message that uses only msg::field (and no msg::disjoint_field), it is normally easy to calculate the necessary message size, since it should be 1 + std::max({FieldsT::DWordIndex...}) (considering we start always from zero), when FieldsT is the template parameter pack of the message fields.

It's not easy, though, to perform this when using messages with msg::disjoint_field because the inner fields are not provided as a type alias and the fields variable is private.

Could a public type alias to FieldsT be provided to make this calculation possible? Or maybe a method that returns a list of DWordIndex of every type?

Add ValueType alias

Same situation as 1, but now for making templates that use the ValueType field. It is avaliable in msg::field but not in msg::disjoint_field, even though it is one of the template arguments.

Could a public alias to T, say, using ValueType = T be provided?

[herculanodavi]

With the #320 merge, now msg::disjoint_field does not compile when part of a msg::message_base because it has no DWordExtent member. #321 fixes that.