Changed `tag.if` and `tag.each` into node methods, removed `tag.fn`, added `repeat`

Without interpolation, there is no need for `tag.fn`, which makes the API smaller:

    ul(tag.each([1,2], li))  =>  ul().each([1,2], li)  =>  "<ul><li>1</li><li>2</li></ul>";
    ul(tag.if(true, 1, 2))   =>  ul().if(true, 1, 2)   =>  "<ul>1</ul>"

The only other case is if you need to to an `if` or `each` outside any other tags.
By creating an "empty tag" you can still do this:

    tag().each([1,2], p)  =>  "<p></p><p></p>"
    tag().if(true, p)     =>  "<p></p>"

There's also a new node method called `repeat`:

    ul.repeat(2, li) => "<ul><li></li><li></li></ul>"

Both `each` and `repeat` can still take in optional iterators.