Now it no longer tries to be this weird hybrid between an owning
and non-owning reference, and is only ever non-owning. This is also
reflected in its interface, for example `StringRef::isNullTerminated`
is now public, and `StringRef::c_str()` has the precondition that it
is true.

Overview of the changes:
* The `StringRef::m_data` member has been completely removed, as it
had no more uses.
* `StringRef::isSubstring()` has been made public and renamed to
`StringRef::isNullTerminated()`, so that the name reflects what the
method actually does.
* `StringRef::currentData()` has been renamed to `StringRef::data()`,
to be in line with common C++ containers and container-alikes.
* `StringRef::c_str()` will no longer silently make copies. It instead
has a precondition that `isNullTerminated()` is true.
* If the user needs a null-terminated string, they should use the
`std::string` conversion operator and call `c_str()` on the resulting
`std::string`.
* Some small optimizations in various places.
* Basic functionality is now `constexpr`.

It never counted characters, only codepoints. If you think these
are interchangeable, you should not touch non-ascii text.

This fixes an issue where a self-assignment of a StringRef copy would point into internally (and now dangling) data.
(now self-assignment check is no longer needed)

This allows reducing the amount of friends needed for its interface
and some extra tricks later.

The bad part is that the pointer can become invalidated via
calls to other StringRef's public methods, but c'est la vie.

This also fixes some tests that were previously failing unnoticed - WTF?