Happening when using clang and templated operators, clang cannot decide
between the operator provided by ReusableStringStream and the one provided
by the value value as both are templates. This is easily solved by calling
the operator<< through the member syntax.

Fixes #1285

This support is based on overriden `std::exception::what` method, so
if an exception does not do so meaningfully, the message is still
pointless.

This is only used as a fallback, both `StringMaker` specialization and
`operator<<` overload have priority..

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