arrow

ARROW-4910: [Rust] [DataFusion] Remove all uses of unimplemented!

This PR removes all uses of `unimplemented!` in DataFusion.

It also refactors to replace two copies of `expr_to_field` and `exprlist_to_fields` with one copy in a new file `optimizer::utils` and also removes some dead code.

There are no functional changes in this PR.

Note that I had already made these changes under the larger PR https://github.com/apache/arrow/pull/3939 but I felt it would be better to break this up into smaller PRs.

Author: Andy Grove <andygrove73@gmail.com>
Author: Andy Grove <andy.grove@rms.com>

Closes #3961 from andygrove/ARROW-4910 and squashes the following commits:

d2e934d0 <Andy Grove> formatting
0039b335 <Andy Grove> Address PR feedback
51da9f0b <Andy Grove> Remove all uses of unimplemented