Rename IUnionEncoder to IEventCodec

[bartelink]

While the encoding support started as a very light wrapper of the TypeShape UnionEncoder (before it was called that!). However

• the base interface does not rely on or special case Unions in any way
• is equally focused on encoding and decoding

So... in V2 we should thus consider renaming IUnionEncoder to IEventCodec. https://github.com/jet/FsCodec/blob/master/src/FsCodec/FsCodec.fs#L31

As this will be a breaking change, it should accompany some meaningful new feature, i.e. #14