Rollup merge of #22311 - lfairy:consistent-fmt, r=alexcrichton

This brings it in line with its namesake in `std::io`.

[breaking-change]

r? @aturon

src/test/run-pass/colorful-write-macros.rs

@@ -21,7 +21,7 @@ struct Foo<'a> {
 
 struct Bar;
 
-impl fmt::Writer for Bar {
+impl fmt::Write for Bar {
     fn write_str(&mut self, _: &str) -> fmt::Result {
         Ok(())
     }
@@ -39,7 +39,7 @@ fn main() {
 
     let mut s = Bar;
     {
-        use std::fmt::Writer;
+        use std::fmt::Write;
         write!(&mut s, "test");
     }
 }

src/test/run-pass/ifmt.rs

@@ -165,9 +165,9 @@ pub fn main() {
 }
 
 // Basic test to make sure that we can invoke the `write!` macro with an
-// io::Writer instance.
+// fmt::Write instance.
 fn test_write() {
-    use std::fmt::Writer;
+    use std::fmt::Write;
     let mut buf = String::new();
     write!(&mut buf, "{}", 3);
     {
@@ -194,7 +194,7 @@ fn test_print() {
 // Just make sure that the macros are defined, there's not really a lot that we
 // can do with them just yet (to test the output)
 fn test_format_args() {
-    use std::fmt::Writer;
+    use std::fmt::Write;
     let mut buf = String::new();
     {
         let w = &mut buf;