Base the CoC on The Go Code, rather than the Contributor's Covenant

[mprelude]

This is a proposal to switch the base for the Code of Conduct to the Go Code, because:

• The Go Code focuses primarily on desired behaviours ('be patient', 'be respectful') and only secondarily on 'avoiding destructive behaviours'.
• The Go Code explicitly notes that it "is not a mechanism for people to silence others with whom they disagree." - this was a major concern on the ML, so codifying that this usage will not be accepted in the CoC itself is a step in the right direction.
• The Go Code encourages everybody to follow the code in all spaces, but limits explicit enforcement to Go spaces.
• The Go Code is far more clear in defining prohibited behaviours.

We should still work to refine the code, but I think that this code is far more likely to achieve a consensus, because it does not suffer from many of the charges levelled at the Contributor covenant.

[hikari-no-yume]

While I personally don't think there's much wrong with the Contributor Covenant, I really like Go's approach. It defining not only a set of enforceable rules, but also positive values, encouraged/discouraged behaviours, and how enforcement works, is a big plus. It means we don't have to (badly?) reinvent the wheel on those issues.