Review Instant API + Duration (#573)

Reviewed both Instant and Duration while working on this, and they
appear to be generally fine.

Made some updates to Instant, but none really seemed to be needed for
Duration.

Author: nekevss

src/builtins/core/instant.rs

@@ -157,7 +157,6 @@ impl From<EpochNanoseconds> for Instant {
 // ==== Private API ====
 
 impl Instant {
-    // TODO: Update to `i128`?
     /// Adds a `TimeDuration` to the current `Instant`.
     ///
     /// Temporal-Proposal equivalent: `AddInstant`.
@@ -272,7 +271,7 @@ impl Instant {
 
     // Utility for converting `Instant` to `i128`.
     pub fn as_i128(&self) -> i128 {
-        self.0 .0
+        self.epoch_nanoseconds().0
     }
 }
 
@@ -369,6 +368,7 @@ impl Instant {
     }
 
     /// Returns an `Instant` by rounding the current `Instant` according to the provided settings.
+    #[inline]
     pub fn round(&self, options: RoundingOptions) -> TemporalResult<Self> {
         let resolved_options = ResolvedRoundingOptions::from_instant_options(options)?;
 
@@ -377,17 +377,21 @@ impl Instant {
     }
 
     /// Returns the `epochMilliseconds` value for this `Instant`.
+    #[inline]
     #[must_use]
     pub fn epoch_milliseconds(&self) -> i64 {
         self.as_i128().div_euclid(1_000_000) as i64
     }
 
-    /// Returns the `epochNanoseconds` value for this `Instant`.
+    /// Returns the [`EpochNanoseconds`] value for this `Instant`.
+    #[inline]
     #[must_use]
     pub fn epoch_nanoseconds(&self) -> &EpochNanoseconds {
         &self.0
     }
 
+    /// Returns a [`ZonedDateTime`] for the current `Instant`.
+    #[inline]
     pub fn to_zoned_date_time_iso_with_provider(
         &self,
         time_zone: TimeZone,
@@ -400,6 +404,7 @@ impl Instant {
 // ==== Instant Provider API ====
 
 impl Instant {
+    /// Returns an RFC9557 IXDTF string representing the current `Instant`.
     pub fn to_ixdtf_string_with_provider(
         &self,
         timezone: Option<TimeZone>,
@@ -410,6 +415,7 @@ impl Instant {
             .map(|x| x.write_to_string().into())
     }
 
+    /// Returns a [`Writeable`] for formatting the current `Instant` in RFC9557's IXDTF.
     pub fn to_ixdtf_writeable_with_provider(
         &self,
         timezone: Option<TimeZone>,