io: add type-specific read/write methods

This seriously improves the serialization/deserialization performance for
several reasons:
 * no time spent in `binary` reflection
 * no memory allocations being made on every read/write
 * uses fast ReadBytes everywhere it's appropriate

It also makes Fixed8 Serializable just for convenience.

pkg/consensus/change_view.go

@@ -15,12 +15,12 @@ var _ payload.ChangeView = (*changeView)(nil)
 
 // EncodeBinary implements io.Serializable interface.
 func (c *changeView) EncodeBinary(w *io.BinWriter) {
-	w.WriteLE(c.timestamp)
+	w.WriteU32LE(c.timestamp)
 }
 
 // DecodeBinary implements io.Serializable interface.
 func (c *changeView) DecodeBinary(r *io.BinReader) {
-	r.ReadLE(&c.timestamp)
+	c.timestamp = r.ReadU32LE()
 }
 
 // NewViewNumber implements payload.ChangeView interface.