The fix leaves an open question: should we keep treating refs
to static array of chars as strings, or should we instead
use `strnlen` to check if it is null-terminated within the buffer

Fixes #1238

Also adds `Predicate` helper function to create `PredicateMatcher`.
Because of limitations in type inference it needs to be explicitly
typed, like so
`Predicate<std::string>([](std::string const& str) { ... })`.
It also takes an optional second argument for description of the
predicate.

It is possible to infer the argument with sufficient TMP, see
https://stackoverflow.com/questions/43560492/how-to-extract-lambdas-return-type-and-variadic-parameters-pack-back-from-gener/43561563#43561563
but I don't think that the magic is worth introducing ATM.

Closes #1236

There are still some holes, e.g. we leave surrogate pairs be
even though they are not a part of valid UTF-8, but this might
be for the better -- WTF-8 does support surrogate pairs inside
text.

Closes #1207

DJGPP cross compiler is targeting DOS which does not support POSIX
signals. Probably for the same reason (targeting DOS) this compiler
does not support wide characters.

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?