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ARROW-5945: [Rust] [DataFusion] Table trait can now be used to build real queries

The Table (DataFrame) trait can now be used to build real queries, since it now supports projection, selection, aggregate, and limit.

Tests were moved from Table to TableImpl and are now more comprehensive.

Not all expressions are supported yet and separate PRs will expand the number of expressions that are supported.

Note that this PR also removes the `optimize` step from `ExecutionContext.create_logical_plan` so it is necessary to explicitly call this now. I was planning on doing this in a separate PR (https://issues.apache.org/jira/browse/ARROW-5948) but ended up needing it here to implement the unit tests.

Author: Andy Grove <andygrove73@gmail.com>

Closes #4875 from andygrove/ARROW-5945 and squashes the following commits:

376759736 <Andy Grove> Table API