Cache Intl.DateTimeFormat to reduce memory footprint

Creating Intl.DateTimeFormat instances in V8 is very slow and memory heavy.
GetFormatterParts and GetCanonicalTimeZoneIdentifier are functions that
are called many times when using Temporal, and they used to create new
instances of Intl.DateTimeFormat for each call. In this commit, we cache them
using the time zone identifier as the key.

It should be noted that doing the same to SystemTimeZone was deliberately
avoided. This is due to the fact that the user's time zone may change during
the execution of a program. An example is calling Temporal.now.zonedDateTimeISO()
which should always output the correct time zone. This shouldn't be a problem
for server-sided code which usually doesn't (or rather, shouldn't) use the
time zone from the environment for calculations.

lib/ecmascript.mjs

@@ -148,6 +148,27 @@ const ES2020 = {
   Type
 };
 
+const IntlDateTimeFormatEnUsCache = new Map();
+
+function getIntlDateTimeFormatEnUsForTimeZone(timeZoneIdentifier) {
+  let instance = IntlDateTimeFormatEnUsCache.get(timeZoneIdentifier);
+  if (instance === undefined) {
+    instance = new IntlDateTimeFormat('en-us', {
+      timeZone: String(timeZoneIdentifier),
+      hour12: false,
+      era: 'short',
+      year: 'numeric',
+      month: 'numeric',
+      day: 'numeric',
+      hour: 'numeric',
+      minute: 'numeric',
+      second: 'numeric'
+    });
+    IntlDateTimeFormatEnUsCache.set(timeZoneIdentifier, instance);
+  }
+  return instance;
+}
+
 export const ES = ObjectAssign({}, ES2020, {
   ToPositiveInteger: ToPositiveInteger,
   ToFiniteInteger: (value) => {
@@ -2081,16 +2102,7 @@ export const ES = ObjectAssign({}, ES2020, {
   GetCanonicalTimeZoneIdentifier: (timeZoneIdentifier) => {
     const offsetNs = ES.ParseOffsetString(timeZoneIdentifier);
     if (offsetNs !== null) return ES.FormatTimeZoneOffsetString(offsetNs);
-    const formatter = new IntlDateTimeFormat('en-us', {
-      timeZone: String(timeZoneIdentifier),
-      hour12: false,
-      year: 'numeric',
-      month: 'numeric',
-      day: 'numeric',
-      hour: 'numeric',
-      minute: 'numeric',
-      second: 'numeric'
-    });
+    const formatter = getIntlDateTimeFormatEnUsForTimeZone(String(timeZoneIdentifier));
     return formatter.resolvedOptions().timeZone;
   },
   GetIANATimeZoneOffsetNanoseconds: (epochNanoseconds, id) => {
@@ -2209,17 +2221,7 @@ export const ES = ObjectAssign({}, ES2020, {
     return result;
   },
   GetFormatterParts: (timeZone, epochMilliseconds) => {
-    const formatter = new IntlDateTimeFormat('en-us', {
-      timeZone,
-      hour12: false,
-      era: 'short',
-      year: 'numeric',
-      month: 'numeric',
-      day: 'numeric',
-      hour: 'numeric',
-      minute: 'numeric',
-      second: 'numeric'
-    });
+    const formatter = getIntlDateTimeFormatEnUsForTimeZone(timeZone);
     // FIXME: can this use formatToParts instead?
     const datetime = formatter.format(new Date(epochMilliseconds));
     const [date, fullYear, time] = datetime.split(/,\s+/);