Remove generic parameters from Output trait.

Rationale:

* In a next commit, we would store a trait reference in
  `PrettyFormatter<T>::out` instead of using a concrete type
  `OutputLocation<T>`.
* If the trait had generic methods, we would get compilation error
  "cannot be made into an object" in that next commit.

Author: aspotashev

library/test/src/console.rs

@@ -57,9 +57,8 @@ impl<T: Write> OutputLocation<T> {
         self.flush()
     }
 
-    pub fn write_plain<S: AsRef<str>>(&mut self, s: S) -> io::Result<()> {
-        let s = s.as_ref();
-        self.write_all(s.as_bytes())?;
+    pub fn write_plain(&mut self, word: &str) -> io::Result<()> {
+        self.write_all(word.as_bytes())?;
         self.flush()
     }
 }

library/test/src/formatters/pretty.rs

@@ -69,6 +69,7 @@ impl<T: Write> PrettyFormatter<T> {
     }
 
     fn write_plain<S: AsRef<str>>(&mut self, s: S) -> io::Result<()> {
+        let s = s.as_ref();
         self.out.write_plain(s)
     }