Feature request: append a directory as the fragment, and turn the last path segment into directory

It is sometimes useful to "join" the directory to the url without specifying the last slash. For example,

let url = Url::parse("https://example.com/")?;
let dir = "dir";
let new_url = url.join_dir(dir)?;  // cool!

instead of

let url = Url::parse("https://example.com/")?;
let dir = "dir";
let new_url = url.join(&(dir + "/"))?;  // ugly!

would be good.

It is also useful to turn the last path segment to directory. For example

let mut url = Url::parse("https://example.com/child");
if url.path.ends_with('/') {   // looks bad
  url.set_path(&(url.path() + "/"));
}

could be replaced with something like

let url = Url::parse("https://example.com/child");
let url = url.ensure_dir();

Start by reading the existing Url::join, path, and set_path entry points to understand URL path resolution and mutation. Add the requested directory-joining and trailing-directory behavior, then run the repository's existing tests and cover the examples in the issue; done means both operations work without manually appending a slash.