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/compiletest/header.rs

@@ -311,7 +311,7 @@ fn parse_exec_env(line: &str) -> Option<(String, String)> {
     parse_name_value_directive(line, "exec-env").map(|nv| {
         // nv is either FOO or FOO=BAR
         let mut strs: Vec<String> = nv
-                                      .splitn(1, '=')
+                                      .splitn(2, '=')
                                       .map(|s| s.to_string())
                                       .collect();