Add support of ES6 Modules

[goremikin]

I would like to request the new feature to add support of ES6 Modules, which are becoming the de-facto standard to package JavaScript code.

This would allow to use fast-memoize natively in modern browsers without any build steps.

[lpellegr]

This looks also like a requirement to fix an issue with formatjs: formatjs/formatjs#1913

[RoXuS]

+1

[longlho]

we ended up publishing a esm fork of this for formatjs