Improve MessagePack model

[jarmuszz]

Current implementation of the MessagePack model allows for creation of malformed objects as the ByteVector class (which underlays most of the types in the model) has a maximum length of Long.MaxValue bytes which is bigger than the specification defines for some types.

This PR proposes following changes:

• Float32(ByteVector) -> Float32(Float)
• Float64(ByteVector) -> Float64(Double)
• Timestamp32(ByteVector) -> Timestamp64(Int)
• Timestamp64(ByteVector, ByteVector) -> Timestamp64(Long) with nanoseconds and seconds fields
• Timestamp96(ByteVector, ByteVector) -> Timestamp96(Int, Long)

Ref. #603

[jarmuszz]

I have currently held back from changing the integer types. UnsignedInt would have to hold a BigInt in order to keep a sign which is not quite performant. We could also bit cast it into a Long but I think it would be confusing for users.

[ybasket]

Looks good to me, let's see if @ybasket has some comments.

No, was only waiting for the small fix you had requested. LGTM