LibJS+AK: Make String.prototype.repeat() way faster

Instead of using a StringBuilder, add a String::repeated(String, N)
overload that takes advantage of knowing it's already all UTF-8.

This makes the following microbenchmark go 4x faster:

    "foo".repeat(100_000_000)

And for single character strings, we can even go 10x faster:

    "x".repeat(100_000_000)

AK/String.cpp

@@ -636,4 +636,21 @@ bool String::equals_ignoring_ascii_case(StringView other) const
     return StringUtils::equals_ignoring_ascii_case(bytes_as_string_view(), other);
 }
 
+String String::repeated(String const& input, size_t count)
+{
+    VERIFY(!Checked<size_t>::multiplication_would_overflow(count, input.bytes().size()));
+    u8* buffer = nullptr;
+    auto data = MUST(Detail::StringData::create_uninitialized(count * input.bytes().size(), buffer));
+
+    if (input.bytes().size() == 1) {
+        memset(buffer, input.bytes().first(), count);
+        return String { move(data) };
+    }
+
+    for (size_t i = 0; i < count; ++i) {
+        memcpy(buffer + (i * input.bytes().size()), input.bytes().data(), input.bytes().size());
+    }
+    return String { data };
+}
+
 }

AK/String.h

@@ -92,6 +92,9 @@ public:
     // Creates a new String with a single code point repeated N times.
     static ErrorOr<String> repeated(u32 code_point, size_t count);
 
+    // Creates a new String from another string, repeated N times.
+    static String repeated(String const&, size_t count);
+
     // Creates a new String by case-transforming this String. Using these methods require linking LibUnicode into your application.
     ErrorOr<String> to_lowercase(Optional<StringView> const& locale = {}) const;
     ErrorOr<String> to_uppercase(Optional<StringView> const& locale = {}) const;

Userland/Libraries/LibJS/Runtime/StringPrototype.cpp

@@ -777,10 +777,7 @@ JS_DEFINE_NATIVE_FUNCTION(StringPrototype::repeat)
         return PrimitiveString::create(vm, String {});
 
     // 6. Return the String value that is made from n copies of S appended together.
-    StringBuilder builder;
+    return PrimitiveString::create(vm, String::repeated(string, n));
-    for (size_t i = 0; i < n; ++i)
-        builder.append(string);
-    return PrimitiveString::create(vm, MUST(builder.to_string()));
 }
 
 // 22.1.3.19 String.prototype.replace ( searchValue, replaceValue ), https://tc39.es/ecma262/#sec-string.prototype.replace