Make String::concat take ArrayView[T]

[tonyfettes]

I was writing a MoonBit code generate to automate the implementation of some trait for tuple in MoonBit, and when I was trying to build the type of tuple (A, B, C, ...), I found there is no way to concat an ArrayView[String] into a String with some separator.

Example code:

let type_names = ["A", "B", ...]

fn build_tuple_type(length : Int) -> String {
  String::concat(type_names[:length], separator=", ")
}

Therefore, I wish there is a way to concat an ArrayView[T] with a separator.