Is your feature request related to a problem or challenge?

This API is used in several optimizer passes to recursively find all column references in an expression

pub fn Expr::to_columns(&self) -> Result<HashSet<Column>, DataFusionError>

However, it requires clone() ing all Columns (which requires allocating and copying a string) even if the caller only needs to check if there is a reference. This is inefficient and leads to a slower optimizer

Describe the solution you'd like

I would like an API that does not require copying

Describe alternatives you've considered

One version might be this (returned hasset has &Expr not `Expr)

impl Expr {
  /// Return all referenced  `Column`s  anywhere in this `Expr` 
  ///
  /// For example 
  /// * `a+b` would return `{a,b}`
  /// * `a + 1 + a` would return `{a}`
  fn column_refs(&self) -> HashSet<&Expr>  { 
  ...
  }

Additional context

No response