char: µoptimise UTF-16 surrogates decoding

According to Godbolt¹, on x86_64 using binary and produces slightly
better code than using subtraction.  Readability of both is pretty
much equivalent so might just as well use the shorter option.

¹ https://rust.godbolt.org/z/9jM3ejbMx

library/core/src/char/decode.rs

@@ -67,7 +67,7 @@ fn next(&mut self) -> Option<Result<char, DecodeUtf16Error>> {
             }
 
             // all ok, so lets decode it.
-            let c = (((u - 0xD800) as u32) << 10 | (u2 - 0xDC00) as u32) + 0x1_0000;
+            let c = (((u & 0x3ff) as u32) << 10 | (u2 & 0x3ff) as u32) + 0x1_0000;
             // SAFETY: we checked that it's a legal unicode value
             Some(Ok(unsafe { from_u32_unchecked(c) }))
         }

library/core/tests/char.rs

@@ -306,6 +306,10 @@ fn check(s: &[u16], expected: &[Result<char, u16>]) {
     }
     check(&[0xD800, 0x41, 0x42], &[Err(0xD800), Ok('A'), Ok('B')]);
     check(&[0xD800, 0], &[Err(0xD800), Ok('\0')]);
+    check(&[0xD800], &[Err(0xD800)]);
+    check(&[0xD840, 0xDC00], &[Ok('\u{20000}')]);
+    check(&[0xD840, 0xD840, 0xDC00], &[Err(0xD840), Ok('\u{20000}')]);
+    check(&[0xDC00, 0xD840], &[Err(0xDC00), Err(0xD840)]);
 }
 
 #[test]