Generated flyweights for unions should throw exception from Builder.build() if kind is not set

[cmebarrow]

Currently calling Builder.wrap then Builder.build results in an empty flyweight (with limit() == offset()). This is dangerous because structure members of type union are defaulted by setting nothing (i.e. just wrap plus build), as are all complex types. It is the responsibility of each (generated) complex type to throw an exception if it does not support defaulting everything.

I discovered this by building TCP extension metadata using TcpBeginEx and setting only localPost, remote address and remote port. Result was a completely empty local address with no bytes on the wire, which cannot possibly be interpreted correctly at the other end.