LibJS: Move ValueTraits to Value.h and add special case for -0.0

This will allow us to use these traits for other hash-based containers
(like Map). This commit also adds a special case for negative zero
values, because while the equality check used same_value_zero which is
negative/positive zero insensitive, the hash was not.

Userland/Libraries/LibJS/Runtime/Set.h

@@ -7,31 +7,12 @@
 #pragma once
 
 #include <AK/HashTable.h>
-#include <LibJS/Runtime/BigInt.h>
 #include <LibJS/Runtime/GlobalObject.h>
 #include <LibJS/Runtime/Object.h>
 #include <LibJS/Runtime/Value.h>
 
 namespace JS {
 
-struct ValueTraits : public Traits<Value> {
-    static unsigned hash(Value value)
-    {
-        VERIFY(!value.is_empty());
-        if (value.is_string())
-            return value.as_string().string().hash();
-
-        if (value.is_bigint())
-            return value.as_bigint().big_integer().hash();
-
-        return u64_hash(value.encoded()); // FIXME: Is this the best way to hash pointers, doubles & ints?
-    }
-    static bool equals(const Value a, const Value b)
-    {
-        return same_value_zero(a, b);
-    }
-};
-
 class Set : public Object {
     JS_OBJECT(Set, Object);
 

Userland/Libraries/LibJS/Runtime/Value.h

@@ -16,6 +16,7 @@
 #include <AK/String.h>
 #include <AK/Types.h>
 #include <LibJS/Forward.h>
+#include <LibJS/Runtime/BigInt.h>
 #include <math.h>
 
 // 2 ** 53 - 1
@@ -376,6 +377,27 @@ Function* species_constructor(GlobalObject&, const Object&, Function& default_co
 Value require_object_coercible(GlobalObject&, Value);
 MarkedValueList create_list_from_array_like(GlobalObject&, Value, AK::Function<Result<void, ErrorType>(Value)> = {});
 
+struct ValueTraits : public Traits<Value> {
+    static unsigned hash(Value value)
+    {
+        VERIFY(!value.is_empty());
+        if (value.is_string())
+            return value.as_string().string().hash();
+
+        if (value.is_bigint())
+            return value.as_bigint().big_integer().hash();
+
+        if (value.is_negative_zero())
+            value = Value(0);
+
+        return u64_hash(value.encoded()); // FIXME: Is this the best way to hash pointers, doubles & ints?
+    }
+    static bool equals(const Value a, const Value b)
+    {
+        return same_value_zero(a, b);
+    }
+};
+
 }
 
 namespace AK {