Phobos version naming

[Andrei Alexandrescu]

I'd like to devise a robust version naming for Phobos. What is the best way?

Currently it's std.v2 but I'm unclear how it goes from there. By semver 
we'd go with std.v2_0_1 and so on.

Walter and others say it's better to have "std2" at top level instead of 
"std.v2" nested inside std. I confess I chose "std.v2" partially because 
it's easier to implement. But we can go either way. What would be best?