Improve performance by preallocating a bit

implementations/rust/preserves/src/value/iolist.rs

@@ -11,14 +11,14 @@ pub struct IOList {
 impl IOList {
     pub fn new() -> Self {
         IOList {
-            chunks: vec![],
+            chunks: Vec::with_capacity(5),
             len: 0,
         }
     }
 
     fn rightmost_chunk(&mut self) -> &mut Vec<u8> {
         if self.chunks.is_empty() {
-            self.chunks.push(vec![]);
+            self.chunks.push(Vec::with_capacity(64))
         }
         self.chunks.last_mut().unwrap()
     }

implementations/rust/preserves/src/value/packed/writer.rs

@@ -55,7 +55,7 @@ impl<W: io::Write> PackedWriter<W> {
         PackedWriter {
             w: write,
             buffer: IOList::new(),
-            items: Vec::new(),
+            items: Vec::with_capacity(16),
             streaming: false,
         }
     }
@@ -117,7 +117,7 @@ impl<W: io::Write> PackedWriter<W> {
     }
 
     pub fn start_seq(&mut self) -> io::Result<()> {
-        self.items.push(Vec::new());
+        self.items.push(Vec::with_capacity(32));
         Ok(())
     }