C++20

[Olly Betts]

Ticket to note things we can take advantage of once we require C++20:

• Heterogeneous comparison lookup for unordered containers - std::unordered_set, etc. StemStopper::stop_words, TermListGroup::termfreq/TermListGroup::get_termfreq, PointType::contains/PointType::get_weight. (We get this for ordered containers in C++14.)

[Olly Betts]

At least for stuff like the above and where the difference isn't exposed in the API headers, we can probably use the __cpp_-prefixed feature macros to opportunistically use the new way with a fall-back to creating a temporary std::string. We can then probe for C++20 in configure and enable if supported by the compiler, with a fallback to C++17.

[Olly Betts]

We can then probe for C++20 in configure and enable if supported by the compiler, with a fallback to C++17.

d3778ed6718d5f638c362e35354947955b78cb52 adds a probe for C++23 with fallback to C++20 then C++17.

We conditionally check for __cpp_lib_associative_heterogeneous_erasure (C++23) currently (also __cpp_lib_associative_heterogeneous_insertion but that's due for C++26). Further such conditionalised uses aren't ABI changing so don't need to block starting a new release series.