Iterator is missing the bare and separator-only variants of addString

[Ichoran]

The old collections have addString in three variants on Iterator:

def addString(b: StringBuilder): StringBuilder
def addString(b: StringBuilder,sep: String): StringBuilder
def addString(b: StringBuilder,start: String,sep: String,end: String): StringBuilder

But the new Iterator has only the last variant.

It should get the other two. In particular,

xs.iterator().mkString == xs.iterator().addString(new StringBuilder).result

should compile and be true.