Use prune with get item, not column (#1976)

This will enable chunk layout the prune chunks with stats.

I will follow up with a PR which converts this logic to use a visitor.

vortex-expr/src/get_item.rs

@@ -8,7 +8,7 @@ use vortex_dtype::FieldName;
 use vortex_error::{vortex_err, VortexResult};
 
 use crate::field::DisplayFieldName;
-use crate::{ExprRef, VortexExpr};
+use crate::{ident, ExprRef, VortexExpr};
 
 #[derive(Debug, Clone, Eq, Hash)]
 #[allow(clippy::derived_hash_with_manual_eq)]
@@ -38,6 +38,10 @@ pub fn get_item(field: impl Into<FieldName>, child: ExprRef) -> ExprRef {
     GetItem::new_expr(field, child)
 }
 
+pub fn get_item_scope(field: impl Into<FieldName>) -> ExprRef {
+    GetItem::new_expr(field, ident())
+}
+
 impl Display for GetItem {
     fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
         write!(f, "{}.{}", self.child, DisplayFieldName(&self.field))

vortex-expr/src/pack.rs

@@ -96,11 +96,13 @@ impl PartialEq<dyn Any> for Pack {
 
 impl Display for Pack {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        let mut f = f.debug_struct("Pack");
-        for (name, value) in self.names.iter().zip_eq(self.values.iter()) {
-            f.field(name, value);
-        }
-        f.finish()
+        f.write_str("{")?;
+        self.names
+            .iter()
+            .zip(&self.values)
+            .format_with(", ", |(name, expr), f| f(&format_args!("{name}: {expr}")))
+            .fmt(f)?;
+        f.write_str("}")
     }
 }