chore: split pruning.rs into four files. (#3465)

1. field_or_identity.rs: just cut-and-paste.
2. relation.rs: cut-and-paste but then I added `map(&self) ->
&HashMap...`
3. pruning_predicate.rs: I fixed some nits but also made this somewhat
larger change:
```diff
@@ -95,26 +32,15 @@ impl PruningPredicate {
         let (expr, required_stats) = convert_to_pruning_expression(original_expr);
         if let Some(lexp) = expr.as_any().downcast_ref::<Literal>() {
             // Is the expression constant false, i.e. prune nothing
-            if lexp
-                .value()
-                .as_bool_opt()
-                .and_then(|b| b.value())
-                .map(|b| !b)
-                .unwrap_or(false)
-            {
-                None
-            } else {
-                Some(Self {
-                    expr,
-                    required_stats,
-                })
+            if lexp.value().as_bool_opt().and_then(|b| b.value()) == Some(false) {
+                return None;
             }
-        } else {
-            Some(Self {
-                expr,
-                required_stats,
-            })
         }
+
+        Some(Self {
+            expr,
+            required_stats,
+        })
     }
```
4. pruning_predicate_rewriter.rs: cut-and-paste but I lifted a type
definition to the top: `type PruningPredicateStats = (ExprRef,
Relation<FieldOrIdentity, Stat>);`

---------

Signed-off-by: Daniel King <[email protected]>

Author: danking