fix: reduce memory held by deferred objects (#96)

`bytes.Buffer` will always overallocate (for performance) and we don't
have a great way of predicting the correct size. So, instead, copy once
when we're done.

deferred.go

@@ -5,8 +5,15 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"sync"
 )
 
+var deferredBufferPool = sync.Pool{
+	New: func() any {
+		return bytes.NewBuffer(nil)
+	},
+}
+
 type Deferred struct {
 	Raw []byte
 }
@@ -24,10 +31,12 @@ func (d *Deferred) MarshalCBOR(w io.Writer) error {
 }
 
 func (d *Deferred) UnmarshalCBOR(br io.Reader) (err error) {
-	// Reuse any existing buffers.
-	reusedBuf := d.Raw[:0]
-	d.Raw = nil
-	buf := bytes.NewBuffer(reusedBuf)
+	buf := deferredBufferPool.Get().(*bytes.Buffer)
+
+	defer func() {
+		buf.Reset()
+		deferredBufferPool.Put(buf)
+	}()
 
 	// Allocate some scratch space.
 	scratch := make([]byte, maxHeaderSize)
@@ -90,6 +99,9 @@ func (d *Deferred) UnmarshalCBOR(br io.Reader) (err error) {
 			return fmt.Errorf("unhandled deferred cbor type: %d", maj)
 		}
 	}
-	d.Raw = buf.Bytes()
+	// Reuse existing buffer. Also, copy to "give back" the allocation in the byte buffer (which
+	// is likely significant).
+	d.Raw = d.Raw[:0]
+	d.Raw = append(d.Raw, buf.Bytes()...)
 	return nil
 }