libutil: add Bytes type

not used anywhere yet, but we'll use this a lot soon for generators that
return file contents, wire protocol fragments, or indeed any byte stream

Change-Id: I01a46f9bf9d75aaf4a5d7662773b99f498862a28

src/libutil/types.hh

@@ -11,6 +11,7 @@
 #include <map>
 #include <variant>
 #include <vector>
+#include <span>
 
 namespace nix {
 
@@ -19,6 +20,10 @@ typedef std::set<std::string> StringSet;
 typedef std::map<std::string, std::string> StringMap;
 typedef std::map<std::string, std::string> StringPairs;
 
+// TODO this should be a std::byte span, but too much of the
+// current codebase predates std::byte and uses char instead
+using Bytes = std::span<const char>;
+
 /**
  * Paths are just strings.
  */