Make serde encoding match new decoding rules

src/modular.rs

@@ -247,6 +247,7 @@ mod tests {
             type U = Uint<BITS, LIMBS>;
             // TODO: Increase cases when perf is better.
             let mut config = Config::default();
+            // BUG: Proptest still runs 5 cases even if we set it to 1.
             config.cases = min(config.cases, if BITS > 500 { 1 } else { 3 });
             proptest!(config, |(a: U, b: U, c: U, m: U)| {
                 // TODO: a^(b+c) = a^b * a^c. Which requires carmichael fn.

src/string.rs

@@ -7,6 +7,8 @@ use thiserror::Error;
 
 // FEATURE: Respect width parameter in formatters.
 
+// TODO: Do we want to write `0` for `BITS == 0`.
+
 impl<const BITS: usize, const LIMBS: usize> Display for Uint<BITS, LIMBS> {
     fn fmt(&self, f: &mut Formatter<'_>) -> FmtResult {
         // Base convert 19 digits at a time

src/support/serde.rs

@@ -18,6 +18,10 @@ impl<const BITS: usize, const LIMBS: usize> Serialize for Uint<BITS, LIMBS> {
     fn serialize<S: Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
         let bytes = self.to_be_bytes_vec();
         if serializer.is_human_readable() {
+            // Special case for zero, which encodes as `0x0`.
+            if BITS == 0 {
+                return serializer.serialize_str("0x0");
+            }
             // OPT: Allocation free method.
             let mut result = String::with_capacity(2 * Self::BYTES + 2);
             result.push_str("0x");