WIP: Add tokomak optimizer

[Dandandan]

Which issue does this PR close?

Closes#440

TODO:

• Decide whether to make this (and the egg dependency) an optional (off by default) feature.
• Add some more tests
• Add support for more expressions to support more queries:
  • dates
  • in
  • between
  • aggregations
  • cast

• Some cleanup

Rationale for this change

See #440 for a longer explanation.
I believe using a optimization library is a great way to accelerate developing & increase the number of optimizations possible.

Also, the more optimizations we add, the more we end up building a own (probably worse) version of a optimization framework.

What changes are included in this PR?

Adds a new optimizer Tokomak that does expression simplification based on rewrite rules.

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #441 (cad2aae) into master (c9ed34c) will decrease coverage by 0.02%.
The diff coverage is 70.78%.

@@ Coverage Diff @@## master #441 +/- ##
==========================================
- Coverage 75.28% 75.26% -0.03% 
==========================================
Files 147 153 +6 Lines 24868 25542 +674 ==========================================
+ Hits 18723 19223 +500 - Misses 6145 6319 +174 

Impacted Files	Coverage Δ
datafusion/src/execution/context.rs	92.08% <ø> (ø)
datafusion/src/optimizer/simplification.rs	70.78% <70.78%> (ø)
datafusion/tests/sql.rs	99.89% <0.00%> (ø)
datafusion-cli/src/main.rs	0.00% <0.00%> (ø)
ballista/rust/core/src/utils.rs	0.00% <0.00%> (ø)
datafusion/src/datasource/csv.rs	72.81% <0.00%> (ø)
ballista/rust/executor/src/main.rs	0.00% <0.00%> (ø)
datafusion/src/physical_plan/mod.rs	78.70% <0.00%> (ø)
ballista/rust/executor/src/execution_loop.rs	0.00% <0.00%> (ø)
ballista/rust/executor/src/flight_service.rs	0.00% <0.00%> (ø)
... and 15 more

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[alamb]

@Dandandan do you still plan to work on this PR?

[houqp]

@Dandandan let me know if you need help with some of the todo list items, happy to help push this PR forward.

[alamb]

Closing stale PRs to keep PR review list manageable. Please reopen if that is a mistake