element.SetBytesCanonical, element.BigEndian and element.LittleEndian

[gbotrel]

This PR introduces SetBytesCanonical

// SetBytesCanonical interprets e as the bytes of a big-endian X-byte integer.
// If e is not a X-byte slice or encodes a value higher than q,
// SetBytesCanonical returns an error.
func (z *Element) SetBytesCanonical(e []byte) error {

element.BigEndian and element.LittleEndian implements a encoding/binary inspired interface:

// A ByteOrder specifies how to convert byte slices into a Element
type ByteOrder interface {
	Element(*[Bytes]byte) (Element, error)
	PutElement(*[Bytes]byte, Element)
	String() string
}