Refactor: Consolidate expression simplification code in simplify_expression.rs

[alamb]

NOTE: This PR just moves code around (it doesn't change logic) so please don't be too scared of its size

Which issue does this PR close?

Re #1160 (more sophisticated expression simplification)

Rationale for this change

There are at least three places where expressions are simplified, with some overlap: const_evaluator.rs, expression_simplifier.rs, and optimizer/util.rs

This makes it hard both to understand what simplifications are applied, as well as what is redundant (e.g. #1164)

What changes are included in this PR?

1. Move ConstEvaluator and Simplifier logic and tests into expression_simplifier.rs.
2. Rename some tests

Are there any user-facing changes?

The location of ConstEvaluator has changed

[alamb]

FYI @rdettai this is one strategy to hopefully make reviews a bit easier: the first PR is mechanical and hopefully quick to review. The second PR can then have logic changes in it without as many mechanical changes.

[alamb]

FYI @houqp / @jgoday / @liukun4515 -- I plan a bit of consolidation in the expression simplification logic. See this PR and the ones linked to it

[liukun4515]

FYI @houqp / @jgoday / @liukun4515 -- I plan a bit of consolidation in the expression simplification logic. See this PR and the ones linked to it

I will review this later.
But the context is too much, I need more time to understand them.
Maybe takes two or three days.

@alamb

[alamb]

I will review this later.

Thank you @liukun4515 -- this PR in particular looks massive but in reality is pretty simple -- I literally cut/pasted code from one module to another

[alamb]

I don't actually think this change requires much review (as it just moves stuff around) so unless anyone has any objections, I'll plan to merge it sometime tomorrow

[liukun4515]

LGTM

[alamb]

Thank you @houqp and @liukun4515