migrate to std::ranges

Move the various seq_ functions, as well as std::(stable_)sort,
std::accumulate, std::transform, std::find, std::find_if to their C++20
std::ranges counterparts.

mu/mu-cmd-extract.cc

@@ -63,7 +63,7 @@ save_parts(const Message& message, const std::string& filename_rx,
 			else if (opts.extract.save_attachments &&
 			    part.looks_like_attachment())
 				return true;
-			else if (seq_some(opts.extract.parts,
+			else if (std::ranges::any_of(opts.extract.parts,
 				     [&](auto&& num){return num==partnum;}))
 				return true;
 			else if (!filename_rx.empty() && part.raw_filename()) {