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/io/size_test.go

@@ -17,7 +17,7 @@ type smthSerializable struct {
 func (*smthSerializable) DecodeBinary(*io.BinReader) {}
 
 func (ss *smthSerializable) EncodeBinary(bw *io.BinWriter) {
-	bw.WriteLE(ss.some)
+	bw.WriteBytes(ss.some[:])
 }
 
 // Mock structure that gives error in EncodeBinary().