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ARROW-3959: [Rust] Add date/time data types

This only adds the date/time types to the DataTypes enum as well as JSON serialization for meta data.

This PR also implements `Schema::to_json`

Author: Andy Grove <andygrove73@gmail.com>

Closes #3340 from andygrove/ARROW-3959 and squashes the following commits:

945498e <Andy Grove> merge from master and implement Hash for DateUnit, TimeUnit, etc.
b05d6a0 <Andy Grove> Merge branch 'master' into ARROW-3959
312885e <Andy Grove> Timestamp now uses TimeUnit
c3e092b <Andy Grove> Merge branch 'master' into ARROW-3959
d289cbb <Andy Grove> improve test
2d36927 <Andy Grove> update unit test
d51bc82 <Andy Grove> fix mistake
f4bbf10 <Andy Grove> Add date/time data types