perf: improve write_fmt to handle simple strings

Per @dtolnay suggestion in serde-rs/serde#2697 (comment) - attempt to speed up performance in the cases of a simple string format without arguments:

```rust
write!(f, "text")  ->  f.write_str("text")
```

Author: nyurik

library/core/src/fmt/mod.rs

@@ -201,14 +201,22 @@ pub trait Write {
         impl<W: Write + ?Sized> SpecWriteFmt for &mut W {
             #[inline]
             default fn spec_write_fmt(mut self, args: Arguments<'_>) -> Result {
-                write(&mut self, args)
+                if let Some(s) = args.as_str() {
+                    self.write_str(s)
+                } else {
+                    write(&mut self, args)
+                }
             }
         }
 
         impl<W: Write> SpecWriteFmt for &mut W {
             #[inline]
             fn spec_write_fmt(self, args: Arguments<'_>) -> Result {
-                write(self, args)
+                if let Some(s) = args.as_str() {
+                    self.write_str(s)
+                } else {
+                    write(&mut self, args)
+                }
             }
         }
 
@@ -1582,8 +1590,13 @@ impl<'a> Formatter<'a> {
     /// assert_eq!(format!("{:0>8}", Foo(2)), "Foo 2");
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[inline]
     pub fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result {
-        write(self.buf, fmt)
+        if let Some(s) = fmt.as_str() {
+            self.buf.write_str(s)
+        } else {
+            write(self.buf, fmt)
+        }
     }
 
     /// Flags for formatting
@@ -2272,8 +2285,13 @@ impl Write for Formatter<'_> {
         self.buf.write_char(c)
     }
 
+    #[inline]
     fn write_fmt(&mut self, args: Arguments<'_>) -> Result {
-        write(self.buf, args)
+        if let Some(s) = args.as_str() {
+            self.buf.write_str(s)
+        } else {
+            write(self.buf, args)
+        }
     }
 }