units: Introduce public hex_u128 function

Introduce a function for parsing a `u128` from hex. Support strings with
and without a `0x` prefix as we do for `hex_u32`.

units/src/parse.rs

@@ -101,6 +101,19 @@ pub fn hex_u32<S: AsRef<str> + Into<String>>(s: S) -> Result<u32, ParseIntError>
     })
 }
 
+/// Parses a `u128` from a hex string.
+///
+/// Input string may or may not contain a `0x` prefix.
+pub fn hex_u128<S: AsRef<str> + Into<String>>(s: S) -> Result<u128, ParseIntError> {
+    let stripped = strip_hex_prefix(s.as_ref());
+    u128::from_str_radix(stripped, 16).map_err(|error| ParseIntError {
+        input: s.into(),
+        bits: 128,
+        is_signed: false,
+        source: error,
+    })
+}
+
 /// Strips the hex prefix off `s` if one is present.
 pub(crate) fn strip_hex_prefix(s: &str) -> &str {
     if let Some(stripped) = s.strip_prefix("0x") {
@@ -201,4 +214,18 @@ mod tests {
         let got = hex_u32("ab").expect("failed to parse non-prefixed hex");
         assert_eq!(got, want);
     }
+
+    #[test]
+    fn parse_u128_from_hex_prefixed() {
+        let want = 3735928559;
+        let got = hex_u128("0xdeadbeef").expect("failed to parse prefixed hex");
+        assert_eq!(got, want);
+    }
+
+    #[test]
+    fn parse_u128_from_hex_no_prefix() {
+        let want = 3735928559;
+        let got = hex_u128("deadbeef").expect("failed to parse non-prefixed hex");
+        assert_eq!(got, want);
+    }
 }