AK: Introduce the new String, replacement for DeprecatedString

DeprecatedString (formerly String) has been with us since the start,
and it has served us well. However, it has a number of shortcomings
that I'd like to address.

Some of these issues are hard if not impossible to solve incrementally
inside of DeprecatedString, so instead of doing that, let's build a new
String class and then incrementally move over to it instead.

Problems in DeprecatedString:

- It assumes string allocation never fails. This makes it impossible
  to use in allocation-sensitive contexts, and is the reason we had to
  ban DeprecatedString from the kernel entirely.

- The awkward null state. DeprecatedString can be null. It's different
  from the empty state, although null strings are considered empty.
  All code is immediately nicer when using Optional<DeprecatedString>
  but DeprecatedString came before Optional, which is how we ended up
  like this.

- The encoding of the underlying data is ambiguous. For the most part,
  we use it as if it's always UTF-8, but there have been cases where
  we pass around strings in other encodings (e.g ISO8859-1)

- operator[] and length() are used to iterate over DeprecatedString one
  byte at a time. This is done all over the codebase, and will *not*
  give the right results unless the string is all ASCII.

How we solve these issues in the new String:

- Functions that may allocate now return ErrorOr<String> so that ENOMEM
  errors can be passed to the caller.

- String has no null state. Use Optional<String> when needed.

- String is always UTF-8. This is validated when constructing a String.
  We may need to add a bypass for this in the future, for cases where
  you have a known-good string, but for now: validate all the things!

- There is no operator[] or length(). You can get the underlying data
  with bytes(), but for iterating over code points, you should be using
  an UTF-8 iterator.

Furthermore, it has two nifty new features:

- String implements a small string optimization (SSO) for strings that
  can fit entirely within a pointer. This means up to 3 bytes on 32-bit
  platforms, and 7 bytes on 64-bit platforms. Such small strings will
  not be heap-allocated.

- String can create substrings without making a deep copy of the
  substring. Instead, the superstring gets +1 refcount from the
  substring, and it acts like a view into the superstring. To make
  substrings like this, use the substring_with_shared_superstring() API.

One caveat:

- String does not guarantee that the underlying data is null-terminated
  like DeprecatedString does today. While this was nifty in a handful of
  places where we were calling C functions, it did stand in the way of
  shared-superstring substrings.

Tests/AK/TestString.cpp

@@ -0,0 +1,102 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibTest/TestCase.h>
+
+#include <AK/String.h>
+#include <AK/StringBuilder.h>
+#include <AK/Try.h>
+#include <AK/Utf8View.h>
+#include <AK/Vector.h>
+
+TEST_CASE(construct_empty)
+{
+    String empty;
+    EXPECT(empty.is_empty());
+    EXPECT_EQ(empty.bytes().size(), 0u);
+
+    auto empty2 = MUST(String::from_utf8(""sv));
+    EXPECT(empty2.is_empty());
+    EXPECT_EQ(empty, empty2);
+    EXPECT_EQ(empty, ""sv);
+}
+
+TEST_CASE(short_strings)
+{
+#ifdef AK_ARCH_64_BIT
+    auto string = MUST(String::from_utf8("abcdefg"sv));
+    EXPECT_EQ(string.is_short_string(), true);
+    EXPECT_EQ(string.bytes().size(), 7u);
+    EXPECT_EQ(string.bytes_as_string_view(), "abcdefg"sv);
+#else
+    auto string = MUST(String::from_utf8("abc"sv));
+    EXPECT_EQ(string.is_short_string(), true);
+    EXPECT_EQ(string.bytes().size(), 3u);
+    EXPECT_EQ(string.bytes_as_string_view(), "abc"sv);
+#endif
+}
+
+TEST_CASE(long_strings)
+{
+    auto string = MUST(String::from_utf8("abcdefgh"sv));
+    EXPECT_EQ(string.is_short_string(), false);
+    EXPECT_EQ(string.bytes().size(), 8u);
+    EXPECT_EQ(string.bytes_as_string_view(), "abcdefgh"sv);
+}
+
+TEST_CASE(substring)
+{
+    auto superstring = MUST(String::from_utf8("Hello I am a long string"sv));
+    auto short_substring = MUST(superstring.substring_from_byte_offset(0, 5));
+    EXPECT_EQ(short_substring, "Hello"sv);
+
+    auto long_substring = MUST(superstring.substring_from_byte_offset(0, 10));
+    EXPECT_EQ(long_substring, "Hello I am"sv);
+}
+
+TEST_CASE(code_points)
+{
+    auto string = MUST(String::from_utf8("🦬🪒"sv));
+
+    Vector<u32> code_points;
+    for (auto code_point : string.code_points())
+        code_points.append(code_point);
+
+    EXPECT_EQ(code_points[0], 0x1f9acu);
+    EXPECT_EQ(code_points[1], 0x1fa92u);
+}
+
+TEST_CASE(string_builder)
+{
+    StringBuilder builder;
+    builder.append_code_point(0x1f9acu);
+    builder.append_code_point(0x1fa92u);
+
+    auto string = MUST(builder.to_string());
+    EXPECT_EQ(string, "🦬🪒"sv);
+    EXPECT_EQ(string.bytes().size(), 8u);
+}
+
+TEST_CASE(ak_format)
+{
+    auto foo = MUST(String::formatted("Hello {}", MUST(String::from_utf8("friends"sv))));
+    EXPECT_EQ(foo, "Hello friends"sv);
+}
+
+TEST_CASE(replace)
+{
+    {
+        auto haystack = MUST(String::from_utf8("Hello enemies"sv));
+        auto result = MUST(haystack.replace("enemies"sv, "friends"sv, ReplaceMode::All));
+        EXPECT_EQ(result, "Hello friends"sv);
+    }
+
+    {
+        auto base_title = MUST(String::from_utf8("anon@courage:~"sv));
+        auto result = MUST(base_title.replace("[*]"sv, "(*)"sv, ReplaceMode::FirstOnly));
+        EXPECT_EQ(result, "anon@courage:~"sv);
+    }
+}