Auto merge of #76017 - JulianKnodt:fmt_fast, r=nagisa

Use less divisions in display u128/i128

This PR is an absolute mess, and I need to test if it improves the speed of fmt::Display for u128/i128, but I think it's correct.
It hopefully is more efficient by cutting u128 into at most 2 u64s, and also chunks by 1e16 instead of just 1e4.

Also I specialized the implementations for uints to always be non-false because it bothered me that it was checked at all

Do not merge until I benchmark it and also clean up the god awful mess of spaghetti.
Based on prior work in #44583

cc: `@Dylan-DPC`

Due to work on `itoa` and suggestion in original issue:
r? `@dtolnay`

Author: bors

library/core/benches/fmt.rs

@@ -108,3 +108,32 @@ fn write_str_macro_debug(bh: &mut Bencher) {
         }
     });
 }
+
+#[bench]
+fn write_u128_max(bh: &mut Bencher) {
+    bh.iter(|| {
+        std::hint::black_box(format!("{}", u128::MAX));
+    });
+}
+
+#[bench]
+fn write_u128_min(bh: &mut Bencher) {
+    bh.iter(|| {
+        let s = format!("{}", 0u128);
+        std::hint::black_box(s);
+    });
+}
+
+#[bench]
+fn write_u64_max(bh: &mut Bencher) {
+    bh.iter(|| {
+        std::hint::black_box(format!("{}", u64::MAX));
+    });
+}
+
+#[bench]
+fn write_u64_min(bh: &mut Bencher) {
+    bh.iter(|| {
+        std::hint::black_box(format!("{}", 0u64));
+    });
+}