[Rust] Move JSON serde code to separate file/module

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Reporter: Andy Grove / @andygrove

_{Note: This issue was originally created as ARROW-2619. Please see the migration documentation for further details.}

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Wes McKinney / @wesm:
Still of interest for 0.14?

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Andy Grove / @andygrove:
No, this isn't a priority for me.

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Neville Dipale / @nevi-me:
@andygrove  is this for datatypes? If so, do we still need to do this? I'm happy with where the code is

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Neville Dipale / @nevi-me:
I'm closing this because the serde code has been updated to comply with the integration format, and I'm fine with where it is now.