Use join instead of open-coding it

src/ffi/schema.rs

@@ -1,5 +1,7 @@
 use std::{collections::BTreeMap, convert::TryInto, ffi::CStr, ffi::CString, ptr};
 
+use itertools::Itertools;
+
 use crate::{
     datatypes::{DataType, Extension, Field, IntervalUnit, Metadata, TimeUnit},
     error::{ArrowError, Result},
@@ -401,13 +403,11 @@ fn to_format(data_type: &DataType) -> String {
             let sparsness = if *is_sparse { 's' } else { 'd' };
             let mut r = format!("+u{}:", sparsness);
             let ids = if let Some(ids) = ids {
-                ids.iter()
-                    .fold(String::new(), |a, b| a + &b.to_string() + ",")
+                ids.iter().map(|x| x.to_string()).join(",")
             } else {
-                (0..f.len()).fold(String::new(), |a, b| a + &b.to_string() + ",")
+                (0..f.len()).map(|x| x.to_string()).join(",")
             };
-            let ids = &ids[..ids.len() - 1]; // take away last ","
-            r.push_str(ids);
+            r.push_str(&ids);
             r
         }
         DataType::Map(_, _) => "+m".to_string(),