Field atoms are duplicated in proc-macro

[evnu]

Currently, the atoms generated for accessing fields in structs within ex_struct.rs, record.rs, etc., are duplicated for both encode and decode. This increases the amount of code generated considerably for larger structs. For example, when using cargo expand on https://github.com/evnu/NifIo/tree/macos-platform-hack, about 200 lines of code are generated for the atoms alone, which is 1/3 of the overall code. See the expanded code

This could be reduced by generating the atoms only once by generating a helper module and referencing that module from encoders and decoders. I would be up to give that a try, but #234 should be merged before this in order to avoid merge issues.