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ARROW-4892: [Rust] [DataFusion] Move SQL parser and planner into SQL module

There are a few small re-org things I would like to do before we release 0.13.0 and this is one of them.

Author: Andy Grove <andygrove73@gmail.com>

Closes #3916 from andygrove/ARROW-4892 and squashes the following commits:

27e11ef <Andy Grove> Move SQL parser and planner into SQL module