Add endian argument to toString method

[Evgeny]

Feature request: add endian (le or be) argument to toString method.

Motivation: working with ed25519 curve from elliptic module there is an inconvenience.

ed25519 uses numbers in little endian order:

const EdDSA = elliptic.eddsa;
const ec = new EdDSA('ed25519');

// hex string in little endian order
const input = '70ee7dd55b7ae8922ed345adc0f5fcc6ccb93c673f38ca7900186efb277337ff';

const num = ec.decodeInt(input);
// utils.intFromLE(bytes)
// new BN(bytes, 'hex', 'le');
// used under hood

// make reduce by n
const reduced = num.umod(ec.curve.n);

There is no simple way to convert reduced number to hex string in little endian order.

// option 1
const output = ec.encodeInt(reduced);
// produces array of numbers in little endian

// option 2
const output = reduced.toString('hex');
// produces hex string but in big endian order

Current solution

const output = reduced.toBuffer('le', 32).toString('hex');

Yep, it works, but it is a bit wired.

Additional context

It would be consistent to have endian argument across all to* method: a.toArray(endian, length), a.toArrayLike(type, endian, length), a.toBuffer(endian, length).

I think it is better to have a look at this issue in case of different bytes order: Wrong padding in toString() function #198