adds implementation of ElementStreamReader (#432)

* adds changes for `ElementStreamReader`
* adds `Display` for `StreamItem`
* adds more unit tests for the reader

src/reader.rs

@@ -17,6 +17,7 @@ use crate::types::decimal::Decimal;
 use crate::types::integer::Integer;
 use crate::types::timestamp::Timestamp;
 use crate::{IonType, RawBinaryReader, RawTextReader};
+use std::fmt::{Display, Formatter};
 
 /// Configures and constructs new instances of [Reader].
 pub struct ReaderBuilder {}
@@ -145,6 +146,29 @@ pub enum StreamItem {
     Nothing,
 }
 
+impl StreamItem {
+    /// If `is_null` is `true`, returns `StreamItem::Value(ion_type)`. Otherwise,
+    /// returns `StreamItem::Null(ion_type)`.
+    pub fn nullable_value(ion_type: IonType, is_null: bool) -> StreamItem {
+        if is_null {
+            StreamItem::Null(ion_type)
+        } else {
+            StreamItem::Value(ion_type)
+        }
+    }
+}
+
+impl Display for StreamItem {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        use StreamItem::*;
+        match self {
+            Value(ion_type) => write!(f, "{}", ion_type),
+            Null(ion_type) => write!(f, "null.{}", ion_type),
+            Nothing => Ok(()),
+        }
+    }
+}
+
 impl<R: RawReader> UserReader<R> {
     pub fn read_raw_symbol(&mut self) -> IonResult<RawSymbolToken> {
         self.raw_reader.read_symbol()

src/text/mod.rs

@@ -1,5 +1,5 @@
 pub mod non_blocking;
-mod parent_container;
+pub(crate) mod parent_container;
 pub(crate) mod parse_result;
 pub(in crate::text) mod parsers;
 pub mod raw_text_reader;