Refactor simplify_expressions and expr_simplifier #3934
Description
Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Currently, the items related to expression simplification are in simplification_expressions.rs and expr_simplifier, which is somewhat messy.
Some structs such as ConstEvaluator and Simplifier are only used in the simplification_expr, but are public in datafusion_optimizer.
Also, there are some util functions that we should also clean them up.
Describe the solution you'd like
I'd like to create a new mod and put all the things above in it.
Describe alternatives you've considered
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