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
43561563 (43561563)
but I don't think that the magic is worth introducing ATM.
Closes#1236
