Updates to instant and its methods (#85)

This PR updates Instant to some of the newer specification changes. 

TLDR of the below:
- Updates `Instant` struct
- Updates diffing ops and adds tests
- Implements `FromStr`.

Primarily, it brings `since` and `until` up to date with both
`temporal_rs` and the specification as a whole (along with at least 2
basic tests for `since` and `until`)

This PR also changes Instant inner nanoseconds from `BigInt` to `i128`.
My thought is to eventually remove all usage of `BigInt` from
`temporal_rs` in general. Primarily motivated by the thought that if
`NormalizedTimeDuration` is using a `i128`, I don't see a huge reason
why `Instant` should differ, unless I'm overlooking something and
someone thinks otherwise.

Finally, it implements `FromStr` for `Instant`. This will allow parsing
of `JsString` in Boa, and would unblock `to_temporal_instant`.

Author: nekevss

src/components/datetime.rs

@@ -64,7 +64,7 @@ impl DateTime {
         offset: f64,
         calendar: Calendar,
     ) -> TemporalResult<Self> {
-        let iso = IsoDateTime::from_epoch_nanos(&instant.nanos, offset)?;
+        let iso = IsoDateTime::from_epoch_nanos(&instant.epoch_nanos, offset)?;
         Ok(Self { iso, calendar })
     }
 

src/components/duration/normalized.rs

@@ -49,6 +49,16 @@ impl NormalizedTimeDuration {
         Self(nanoseconds)
     }
 
+    /// Equivalent to 7.5.27 NormalizedTimeDurationFromEpochNanosecondsDifference ( one, two )
+    pub(crate) fn from_nanosecond_difference(one: i128, two: i128) -> TemporalResult<Self> {
+        let result = one - two;
+        if result.abs() > MAX_TIME_DURATION {
+            return Err(TemporalError::range()
+                .with_message("normalizedTimeDuration exceeds maxTimeDuration."));
+        }
+        Ok(Self(result))
+    }
+
     // NOTE: `days: f64` should be an integer -> `i64`.
     /// Equivalent: 7.5.23 Add24HourDaysToNormalizedTimeDuration ( d, days )
     #[allow(unused)]

src/components/duration/time.rs

@@ -23,7 +23,7 @@ use num_traits::{Euclid, FromPrimitive, MulAdd};
 /// [spec]: https://tc39.es/proposal-temporal/#sec-temporal-time-duration-records
 /// [field spec]: https://tc39.es/proposal-temporal/#sec-properties-of-temporal-duration-instances
 #[non_exhaustive]
-#[derive(Debug, Default, Clone, Copy)]
+#[derive(Debug, Default, Clone, Copy, PartialEq, PartialOrd)]
 pub struct TimeDuration {
     /// `TimeDuration`'s internal hour value.
     pub hours: f64,