Add NTRU back

[dstebila]

We received a proposal from Akira Nagai and colleagues at NTT to re-add NTRU to liboqs.

Presentation from NTT to add NTRU back to liboqs

[SWilson4]

I would welcome the addition of an algorithm in which there is clear community interest and for which we have a committed maintainer—let's do it. I would only add the caveat that, should NTT cease to provide maintenance for the integration in the future, liboqs reserves the right to drop support for NTRU (with adequate warning for our users, giving somebody else the opportunity to step up).