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ARROW-9618: [Rust] [DataFusion] Made it easier to write optimizers

This PR adds 5 auxiliary functions to the optimizer package that allow us to more easily write optimizers and re-writes the optimizers using them.

This PR assumes that the majority of the optimizations will:

* recurse logical expressions and re-write them (currently we have the type coercion)
* recurse logical plans and re-write them (currently we have the projection pushdown)

To transverse and re-write expressions, this PR introduces two functions:

```
/// Returns all expressions composing the expression.
/// E.g. if the expression is "(a + 1) + 1", it returns ["a + 1", "1"] (as Expr objects)
fn expr_expressions(expr: &Expr) -> Result<Vec<&Expr>>

/// returns a new expression where the expressions in expr are replaced by the ones in `expr`.
/// This is used in conjunction with ``expr_expressions`` to re-write expressions.
pub fn from_expression(expr: &Expr, expressions: &Vec<Expr>) -> Result<Expr> {
```

these are used on an optimizer as follows:

```
let mut expressions = expr_expressions(expr)?;

# recurse on the expression
let mut expressions = expressions.iter().map(|e| recursion(e)).collect()

# modify `expressions` (optimizer's code)

utils::from_expression(expr, &expressions)
```

Likewise, this PR introduces 3 functions for the plans:

```
/// returns all expressions in the logical plan.
pub fn expressions(plan: &LogicalPlan) -> Vec<Expr>

/// returns all inputs in the logical plan
pub fn inputs(plan: &LogicalPlan) -> Vec<&LogicalPlan>

/// Returns a new logical plan based on the original one with inputs and expressions replaced
pub fn from_plan(
    plan: &LogicalPlan,
    expr: &Vec<Expr>,
    inputs: &Vec<LogicalPlan>,
) -> Result<LogicalPlan>
```

Overall, these make writing optimizers a much developer-friendlier experience because expressions or plans that the optimizer does not care can just be left alone.

Closes #7879 from jorgecarleitao/simpler

Authored-by: Jorge C. Leitao <jorgecarleitao@gmail.com>
Signed-off-by: Andy Grove <andygrove73@gmail.com>