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ARROW-6944: [Rust] Add String, FixedSizeBinary types

This PR creates a `StringArray` which behaves exactly like the current `BinaryArray`, then restricts the `BinaryArray` to dealing with binary data only. It also adds a `FixedSizeBinaryArray` which is backed by a `FixedSizeListArray`.

Closes #5722 from nevi-me/RROW-6944 and squashes the following commits:

94888c43f <Neville Dipale> fix test failures
d9411b844 <Neville Dipale> reapply on array_ops
14d593c84 <Neville Dipale> address review comments
6f8d6eb37 <Neville Dipale> add tests and clean up implementation
6c6bd50c2 <Neville Dipale> fix examples and benches
a984a6b7c <Neville Dipale> replace use of Binary with String in datafusion
d03197971 <Neville Dipale> ARROW-6944:  Add String, FixedSizeBinary types

Authored-by: Neville Dipale <nevilledips@gmail.com>
Signed-off-by: Neville Dipale <nevilledips@gmail.com>