head: use std::io::copy() with TakeLines reader

Replace the custom `split::walk_lines()` function with a call to
`std::io::copy()`, using a new `TakeLines` reader as the source and
`stdout` as the destination. The `TakeLines` reader is an adaptor that
scans the bytes being read for line ending characters and stops the
reading after a given number of lines has been read (similar to the
`std::io::Take` adaptor).

This change

* makes the `read_n_lines()` function more concise,
* allows it to mirror the implementation of `read_n_bytes()`,
* increases the speed of `head -n NUM`.

Cargo.lock

@@ -2507,6 +2507,7 @@ name = "uu_head"
 version = "0.0.7"
 dependencies = [
  "clap",
+ "memchr 2.4.0",
  "uucore",
  "uucore_procs",
 ]