stdin handling is an unfriendly API

[epage]

Based on #71

The automatic reaction is to write something like

let mut cmd = Command::new("cat");
cmd
    .with_stdin()
    .buffer("42");
let assert = cmd.assert();
assert
  .success()
  .stdout("42");

When you really need to write

 let mut cmd = Command::new("cat");
 cmd
     .arg("-et");
 cmd
     .with_stdin()
     .buffer("42")
     .assert()
     .stdout("42");

The problem is an issue with &mut builders (we can't chain off the original cmd), assuming with_stdin will act like &mut builder function when in reality you should only be acting on it at that point, etc.

Note: The lib.rs documentation still has the broken approach

[epage]

Ideas

• Switch with_stdin to take ownership to ensure it is use correctly
• Add must_use to things so people get compiler errors

Either way, the need to split this into three statements is not resolved.