Serde support for serializing and deserializing binary blobs in XML files

[elrnv]

This is another attempt to support binary blobs in serde, needed for vtkio.

This attempts to address #623 While initial support was added #783 this adds it for the serde api.

This PR also adjusts how trimming is handled in the serde API since this is important when dealing with binary. I tried to be consistent with the event based Reader/Writer APIs.

Please inspect the test failures to evaluate if this makes sense.