Allow calendar.fields() to return an iterable

The fields() methods of the built-in Temporal calendars will only ever
return arrays, but this allows custom calendars to return an iterable
(such as a Set.)

See: #1427

Author: ptomato

index.d.ts

@@ -686,7 +686,7 @@ export namespace Temporal {
         | /** @deprecated */ 'day'
       >
     ): Temporal.Duration;
-    fields?(fields: Array<string>): Array<string>;
+    fields?(fields: Array<string>): Iterable<string>;
     mergeFields?(fields: Record<string, unknown>, additionalFields: Record<string, unknown>): Record<string, unknown>;
     toString(): string;
   }
@@ -769,7 +769,7 @@ export namespace Temporal {
         | /** @deprecated */ 'day'
       >
     ): Temporal.Duration;
-    fields(fields: Array<string>): Array<string>;
+    fields(fields: Array<string>): Iterable<string>;
     mergeFields(fields: Record<string, unknown>, additionalFields: Record<string, unknown>): Record<string, unknown>;
     toString(): string;
   }

lib/ecmascript.mjs

@@ -1735,7 +1735,12 @@ export const ES = ObjectAssign({}, ES2020, {
   CalendarFields: (calendar, fieldNames) => {
     const fields = ES.GetMethod(calendar, 'fields');
     if (fields !== undefined) fieldNames = ES.Call(fields, calendar, [fieldNames]);
-    return ES.CreateListFromArrayLike(fieldNames, ['String']);
+    const result = [];
+    for (const name of fieldNames) {
+      if (ES.Type(name) !== 'String') throw new TypeError('bad return from calendar.fields()');
+      ArrayPrototypePush.call(result, name);
+    }
+    return result;
   },
   CalendarMergeFields: (calendar, fields, additionalFields) => {
     const mergeFields = ES.GetMethod(calendar, 'mergeFields');