String concenatation

[annevk]

How about:

To string concatenate a list list with an optional separator separator, run these steps:

1. If list is empty, then return the empty string.
2. Assert: list only contains strings.
3. If separator is not given, then set separator to the empty string.
4. Assert: separator is a string.
5. Return a string whose contents are list's items, in order, separated from each other by separator.

Needed for whatwg/dom#555 and HTML; possibly other places. (URL concatenates bytes as well, but we don't have generics so...)

cc @Ms2ger

[domenic]

I'd minorly prefer "a list of strings list with an optional separator string separator" then remove the asserts.

I think you could also just define it as "concatenate" with for=string or similar.