Auto merge of rust-lang#82576 - gilescope:to_string, r=Amanieu

i8 and u8::to_string() specialisation (far less asm).

Take 2. Around 1/6th of the assembly to without specialisation.

https://godbolt.org/z/bzz8Mq

(partially fixes rust-lang#73533 )

library/alloc/src/string.rs

@@ -2288,6 +2288,47 @@ impl ToString for char {
     }
 }
 
+#[stable(feature = "u8_to_string_specialization", since = "1.999.0")]
+impl ToString for u8 {
+    #[inline]
+    fn to_string(&self) -> String {
+        let mut buf = String::with_capacity(3);
+        let mut n = *self;
+        if n >= 10 {
+            if n >= 100 {
+                buf.push((b'0' + n / 100) as char);
+                n %= 100;
+            }
+            buf.push((b'0' + n / 10) as char);
+            n %= 10;
+        }
+        buf.push((b'0' + n) as char);
+        buf
+    }
+}
+
+#[stable(feature = "i8_to_string_specialization", since = "1.999.0")]
+impl ToString for i8 {
+    #[inline]
+    fn to_string(&self) -> String {
+        let mut buf = String::with_capacity(4);
+        if self.is_negative() {
+            buf.push('-');
+        }
+        let mut n = self.unsigned_abs();
+        if n >= 10 {
+            if n >= 100 {
+                buf.push('1');
+                n -= 100;
+            }
+            buf.push((b'0' + n / 10) as char);
+            n %= 10;
+        }
+        buf.push((b'0' + n) as char);
+        buf
+    }
+}
+
 #[stable(feature = "str_to_string_specialization", since = "1.9.0")]
 impl ToString for str {
     #[inline]