Add a private flag so generated structs aren't visible outside the package

[rikonor]

Hi,

I noticed that the generated code will contain many exported things that don't necessarily need to be exported.

A few examples are:

DeserializeAvroContainerBlock
DeserializeAvroContainerHeader
AvroContainerBlock
AvroContainerHeader
Magic
Sync

Should those be left as is or possibly made private?