chore: checkpoint before Python removal

vendor/serde_json/src/lexical/digit.rs

@@ -0,0 +1,18 @@
+// Adapted from https://github.com/Alexhuszagh/rust-lexical.
+
+//! Helpers to convert and add digits from characters.
+
+// Convert u8 to digit.
+#[inline]
+pub(crate) fn to_digit(c: u8) -> Option<u32> {
+    (c as char).to_digit(10)
+}
+
+// Add digit to mantissa.
+#[inline]
+pub(crate) fn add_digit(value: u64, digit: u32) -> Option<u64> {
+    match value.checked_mul(10) {
+        None => None,
+        Some(n) => n.checked_add(digit as u64),
+    }
+}