Reject datetime when fraction digits are too large (#229)

We were unwrapping rather than rejecting on the new `to_nanosecond`
method for `Fraction`.

Context:
`to_nanosecond` returns an `Option<u32>`, which represents the
nanoseconds value when the digits are < 9, but returns `None` if the
digits exceed 9.

I think this mostly points towards a doc improvement needed in `ixdtf`
that also links to `to_truncated_nanosecond` to show the difference
between the two cases.

Author: nekevss

src/iso.rs

@@ -635,11 +635,17 @@ impl IsoTime {
     /// Returns an `IsoTime` based off parse components.
     pub(crate) fn from_time_record(time_record: TimeRecord) -> TemporalResult<Self> {
         let second = time_record.second.clamp(0, 59);
-        let (millisecond, rem) = time_record
+        let fractional_seconds = time_record
             .fraction
-            .and_then(|x| x.to_nanoseconds())
-            .map(|x| x.div_rem_euclid(&1_000_000))
-            .unwrap_or((0, 0));
+            .map(|x| {
+                x.to_nanoseconds().ok_or(
+                    TemporalError::range().with_message("fractional seconds exceeds nine digits."),
+                )
+            })
+            .transpose()?
+            .unwrap_or(0);
+
+        let (millisecond, rem) = fractional_seconds.div_rem_euclid(&1_000_000);
         let (micros, nanos) = rem.div_rem_euclid(&1_000);
 
         Self::new(