LibJS: Start implementing Temporal.Instant

Just like the initial Temporal.TimeZone commit, this patch adds the
Instant object itself, its constructor and prototype (currently empty),
and two required abstract operations.

Userland/Libraries/LibJS/Runtime/Temporal/Instant.cpp

@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 2021, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibCrypto/BigInt/SignedBigInteger.h>
+#include <LibJS/Runtime/AbstractOperations.h>
+#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/Temporal/Instant.h>
+#include <LibJS/Runtime/Temporal/InstantConstructor.h>
+
+namespace JS::Temporal {
+
+// 8 Temporal.Instant Objects, https://tc39.es/proposal-temporal/#sec-temporal-instant-objects
+Instant::Instant(BigInt& nanoseconds, Object& prototype)
+    : Object(prototype)
+    , m_nanoseconds(nanoseconds)
+{
+}
+
+void Instant::visit_edges(Cell::Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+
+    visitor.visit(&m_nanoseconds);
+}
+
+// 8.5.1 IsValidEpochNanoseconds ( epochNanoseconds ), https://tc39.es/proposal-temporal/#sec-temporal-isvalidepochnanoseconds
+bool is_valid_epoch_nanoseconds(BigInt const& epoch_nanoseconds)
+{
+    // 1. Assert: Type(epochNanoseconds) is BigInt.
+
+    // 2. If epochNanoseconds < −86400ℤ × 10^17ℤ or epochNanoseconds > 86400ℤ × 10^17ℤ, then
+    if (epoch_nanoseconds.big_integer() < INSTANT_NANOSECONDS_MIN || epoch_nanoseconds.big_integer() > INSTANT_NANOSECONDS_MAX) {
+        // a. Return false.
+        return false;
+    }
+
+    // 3. Return true.
+    return true;
+}
+
+// 8.5.2 CreateTemporalInstant ( epochNanoseconds [ , newTarget ] ), https://tc39.es/proposal-temporal/#sec-temporal-createtemporalinstant
+Object* create_temporal_instant(GlobalObject& global_object, BigInt& epoch_nanoseconds, FunctionObject* new_target)
+{
+    auto& vm = global_object.vm();
+
+    // 1. Assert: Type(epochNanoseconds) is BigInt.
+
+    // 2. Assert: ! IsValidEpochNanoseconds(epochNanoseconds) is true.
+    VERIFY(is_valid_epoch_nanoseconds(epoch_nanoseconds));
+
+    // 3. If newTarget is not present, set it to %Temporal.Instant%.
+    if (!new_target)
+        new_target = global_object.temporal_instant_constructor();
+
+    // 4. Let object be ? OrdinaryCreateFromConstructor(newTarget, "%Temporal.Instant.prototype%", « [[InitializedTemporalInstant]], [[Nanoseconds]] »).
+    // 5. Set object.[[Nanoseconds]] to epochNanoseconds.
+    auto* object = ordinary_create_from_constructor<Instant>(global_object, *new_target, &GlobalObject::temporal_instant_prototype, epoch_nanoseconds);
+    if (vm.exception())
+        return {};
+
+    // 6. Return object.
+    return object;
+}
+
+}