AK: Construct Strings from StringBuilder without re-allocating the data

Currently, invoking StringBuilder::to_string will re-allocate the string
data to construct the String. This is wasteful both in terms of memory
and speed.

The goal here is to simply hand the string buffer over to String, and
let String take ownership of that buffer. To do this, StringBuilder must
have the same memory layout as Detail::StringData. This layout is just
the members of the StringData class followed by the string itself.

So when a StringBuilder is created, we reserve sizeof(StringData) bytes
at the front of the buffer. StringData can then construct itself into
the buffer with placement new.

Things to note:
* StringData must now be aware of the actual capacity of its buffer, as
  that can be larger than the string size.
* We must take care not to pass ownership of inlined string buffers, as
  these live on the stack.

AK/StringBase.cpp

@@ -90,6 +90,19 @@ bool StringBase::operator==(StringBase const& other) const
     return bytes() == other.bytes();
 }
 
+void StringBase::replace_with_string_builder(StringBuilder& builder)
+{
+    if (builder.length() <= MAX_SHORT_STRING_BYTE_COUNT) {
+        return replace_with_new_short_string(builder.length(), [&](Bytes buffer) {
+            builder.string_view().bytes().copy_to(buffer);
+        });
+    }
+
+    destroy_string();
+
+    m_data = &StringData::create_from_string_builder(builder).leak_ref();
+}
+
 ErrorOr<Bytes> StringBase::replace_with_uninitialized_buffer(size_t byte_count)
 {
     if (byte_count <= MAX_SHORT_STRING_BYTE_COUNT)