Std is moving to leanprover-community

Stream: mathlib4

[Mario Carneiro]

Re-sharing this for visibility with those not following #std4 : https://leanprover.zulipchat.com/#narrow/stream/348111-std4/topic/Std.20-.3E.20Boost.20migration/near/435086994 . In particular, please vote on the naming question, we'd like to have it settled before the move if possible.