refactor: Split schema module to multi file module (#989)

close #981

- `Schema` and `SchemaBuilder` are kept in `schema/mod.rs`
- All other individual functions (along with the tests) are moved into a
new file.

Signed-off-by: xxchan <[email protected]>

Author: xxchan

crates/iceberg/src/expr/predicate.rs

@@ -943,7 +943,7 @@ mod tests {
         assert_eq!(result, expected);
     }
 
-    fn table_schema_simple() -> SchemaRef {
+    pub fn table_schema_simple() -> SchemaRef {
         Arc::new(
             Schema::builder()
                 .with_schema_id(1)