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/crypto/keys/publickey.go

@@ -168,7 +168,7 @@ func (p *PublicKey) DecodeBinary(r *io.BinReader) {
 	var x, y *big.Int
 	var err error
 
-	r.ReadLE(&prefix)
+	prefix = uint8(r.ReadByte())
 	if r.Err != nil {
 		return
 	}