std: Changing the meaning of the count to splitn

This commit is an implementation of [RFC 979][rfc] which changes the meaning of
the count parameter to the `splitn` function on strings and slices. The
parameter now means the number of items that are returned from the iterator, not
the number of splits that are made.

[rfc]: https://github.com/rust-lang/rfcs/pull/979

Closes #23911
[breaking-change]

src/librustc_driver/pretty.rs

@@ -74,7 +74,7 @@ pub enum PpMode {
 pub fn parse_pretty(sess: &Session,
                     name: &str,
                     extended: bool) -> (PpMode, Option<UserIdentifiedItem>) {
-    let mut split = name.splitn(1, '=');
+    let mut split = name.splitn(2, '=');
     let first = split.next().unwrap();
     let opt_second = split.next();
     let first = match (first, extended) {