Now it actually only exposes methods to allocate uninitialized storage
and to create substring with a shared superstring. All the details of
the memory layout are fully encapsulated.
The idea is to eventually get rid of protected state in StringBase. To
do this, we first need to remove all references to m_data and
m_short_string from String.
This starts separating memory management of string data and string
utilities like `String::formatted`. This would also allow to reuse the
same storage in `DeprecatedString` in the future.
