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`.

This way it is explicit when there is a `StringRef` -> `std::string`
conversion and makes it easier to look for allocations that could
be avoided.

Doing this has already removed one allocation per registered test
case, as there was a completely pointless `StringRef` -> `std::string`
conversion when parsing tags of a test case.

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)

Thanks to Alex Tkachenko for spotting it.

Now takes and returns StringRefs