Use TextMarshaler interface to simplify eth.Address and eth.Data

[ryanschneider]

Not directly related to #61, but we could also simplify with the TextMarshaler and TextUnmarshaler interface, so we don't have to deal with json.Marshal/Unmarshal in our functions for the base64 encoding/decoding.

Those text marshalers would focus solely on validating and returning a canonical representation of an address, which would be compatible with other encoding formats.

Before

func (a *Address) UnmarshalJSON(data []byte) error {
	// We'll keep the checksummed string in memory so we can use it for internal representations
	str, err := unmarshalHex(data, 20, "data")
	str = ToChecksumAddress(str)
	if err != nil {
		return err
	}
	*a = Address(str)
	return nil
}

func (a Address) MarshalJSON() ([]byte, error) {
	// Seems like geth and parity both return the lower-cased string rather than the checksummed one
	s := strings.ToLower(string(a))
	return json.Marshal(&s)
}

After

func (a *Address) UnmarshalText(text []byte) error {
	str, err := validateHex(string(text), 20, "data")
	if err != nil {
		return err
	}
	str = ToChecksumAddress(str)
	*a = Address(str)
	return nil
}

func (a Address) MarshalText() ([]byte, error) {
	return bytes.ToLower([]byte(a)), nil
}

Originally posted by @basgys in #61 (comment)