add Iter[String]::join

[Yoorkin]

String::concat was a mistake. In the self parameter, the types in the type constructor can be a concrete type like String. After discussing offline, we will move to Iter[String]::join and make String::concat a function. The join method will be added later because users can always use other methods instead.

Originally posted by @Yoorkin in #1249 (comment)

[yj-qin]

Related issue #1171