Add BigInt support and Unicode handling

[broofa]

• Adds support for the fnv1a.bigInt() api suggested in Support higher bit variants #3.
• Also deals with unicode strings in a way that's consistent with other fnv1a hash implementations. [Edit: This changes the hash for any string that has unicode in it, hence the BREAKING CHANGE action.]

mocha and ava both puke if you try to do BigInts in them, so I've had to resort to testing with the native assert module. That may or may not be adequate for you. And xo doesn't recognize bigint literals, nor does ESLint, so I've had to remove the linting step from npm test.

Note: I ended up incorporating some of @ShynRou's work for BigInt support. He should get the IssueHunt bounty for this.

P.S. FWIW, the legacy implementation (32 bit Numbers) is ~9X faster than the BigInt counterpart. https://jsperf.com/fnv1a-number-vs-bigint-perf

Fixes #2
Fixes #3
Closes #7
Closes #4

[broofa]

[sindresorhus]

@broofa Really sorry for leaving this for so long. Real-life and a way too long PR queue came in the way...

[sindresorhus]

ava both puke if you try to do BigInts in them,

I just tested with latest version of AVA and BigInt and 1n works fine now. Can you bring back AVA as testing?

And xo doesn't recognize bigint literals, nor does ESLint, so I've had to remove the linting step from npm test.

XO and ESLint support it now. Can you bring back XO?

[sindresorhus]

You need to update index.d.ts too.

[broofa]

@sindresorhus Updated to use xo and ava. I need some guidance on the following, however:

1. I'm not a TypeScript person, so have no idea what I'm doing in index.d.ts
2. I updated to xo@latest, but it's still failing due to "BigInt is not defined. no-undef". (See failing Travis tests)

[sindresorhus]

Thank you for your work on this, @broofa 👍

[broofa]