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ARROW-4589: [Rust] Projection push down query optimizer rule

This PR adds the first query optimizer rule, which rewrites a logical plan to push the projection down to the TableScan.

Once this is merged, I will create a follow up PR to integrate this into the query engine so that only the necessary columns are loaded from disk.

Author: Andy Grove <andygrove73@gmail.com>

Closes #3664 from andygrove/ARROW-4589-wip and squashes the following commits:

b876f28 <Andy Grove> revert formatting change that broke the tests
2051deb <Andy Grove> formatting comments and test strings to be < 90 columns wide
8effde3 <Andy Grove> Address PR feedback, fix bug, add extra unit test
ecdd32a <Andy Grove> refactor code to reduce duplication
6229b32 <Andy Grove> refactor code to reduce duplication
f959500 <Andy Grove> implement projection push down for rest of logical plan variants
5fd5382 <Andy Grove> implement collect_expr and rewrite_expr for all expression types
bd49f17 <Andy Grove> improve error handling
92918dd <Andy Grove> Implement projection push-down for selection and make projection deterministic
a80cfdf <Andy Grove> Implement mapping and expression rewrite logic
26fd3b4 <Andy Grove> revert change
d7c4822 <Andy Grove> formatting and add assertion to test
e81af14 <Andy Grove> Roughing out projection push down rule