perf: serialize 128-bit integers via itoa

This optimizes the `serialize_i128` and `serialize_u128`
implementations by having them use `itoa`, as all of the other
integer implementations do. This avoids going through the `Display`
implementation of `u128`/`i128` and an intermediate allocation, which
is necessary because of how `collect_str` is currently implemented.

The current implementation seems to be a remnant of the pre-1.0 `itoa`
days, which didn't support `u128` and `i128`.

PR #401

Author: paolobarbolini

src/serializer.rs

@@ -68,7 +68,8 @@ impl<'a, 'w, W: io::Write> Serializer for &'a mut SeRecord<'w, W> {
 
     serde_if_integer128! {
         fn serialize_i128(self, v: i128) -> Result<Self::Ok, Self::Error> {
-            self.collect_str(&v)
+            let mut buffer = itoa::Buffer::new();
+            self.wtr.write_field(buffer.format(v))
         }
     }
 
@@ -94,7 +95,8 @@ impl<'a, 'w, W: io::Write> Serializer for &'a mut SeRecord<'w, W> {
 
     serde_if_integer128! {
         fn serialize_u128(self, v: u128) -> Result<Self::Ok, Self::Error> {
-            self.collect_str(&v)
+            let mut buffer = itoa::Buffer::new();
+            self.wtr.write_field(buffer.format(v))
         }
     }