AK: Avoid temporary Vector<u8> when encoding Base64

Instead, decode directly into an uninitialized StringData object.
This avoids the redundant vector allocation + memory copy.

Author: awesomekling

AK/Base64.cpp

@@ -83,16 +83,20 @@ static ErrorOr<ByteBuffer> decode_base64_impl(StringView input, LastChunkHandlin
 
 static ErrorOr<String> encode_base64_impl(StringView input, simdutf::base64_options options)
 {
-    Vector<u8> output;
-    TRY(output.try_resize(simdutf::base64_length_from_binary(input.length(), options)));
+    if (input.is_empty())
+        return String {};
+
+    u8* buffer = nullptr;
+    auto output = TRY(AK::Detail::StringData::create_uninitialized(
+        simdutf::base64_length_from_binary(input.length(), options), buffer));
 
     simdutf::binary_to_base64(
         input.characters_without_null_termination(),
         input.length(),
-        reinterpret_cast<char*>(output.data()),
+        reinterpret_cast<char*>(buffer),
         options);
 
-    return String::from_utf8_without_validation(output);
+    return String { move(output) };
 }
 
 ErrorOr<ByteBuffer> decode_base64(StringView input, LastChunkHandling last_chunk_handling)

AK/StringBase.h

@@ -52,6 +52,8 @@ class StringBase {
         other.m_impl = { .short_string = ShortString::create_empty() };
     }
 
+    explicit StringBase(NonnullRefPtr<Detail::StringData const>);
+
     StringBase& operator=(StringBase&&);
     StringBase& operator=(StringBase const&);
 
@@ -123,8 +125,6 @@ class StringBase {
     friend class ::AK::FlyString;
     friend struct ::AK::Detail::ShortString;
 
-    explicit StringBase(NonnullRefPtr<Detail::StringData const>);
-
     explicit constexpr StringBase(nullptr_t)
         : m_impl { .data = nullptr }
     {