Simplify ContentAddress

Whereas `ContentAddressWithReferences` is a sum type complex because different
varieties support different notions of reference, and
`ContentAddressMethod` is a nested enum to support that,
`ContentAddress` can be a simple pair of a method and hash.

`ContentAddress` does not need to be a sum type on the outside because
the choice of method doesn't effect what type of hashes we can use.

Co-Authored-By: Cale Gibbard <cgibbard@gmail.com>

perl/lib/Nix/Store.xs

@@ -294,10 +294,8 @@ SV * makeFixedOutputPath(int recursive, char * algo, char * hash, char * name)
             auto h = Hash::parseAny(hash, parseHashType(algo));
             auto method = recursive ? FileIngestionMethod::Recursive : FileIngestionMethod::Flat;
             auto path = store()->makeFixedOutputPath(name, FixedOutputInfo {
-                .hash = {
-                    .method = method,
-                    .hash = h,
-                },
+                .method = method,
+                .hash = h,
                 .references = {},
             });
             XPUSHs(sv_2mortal(newSVpv(store()->printStorePath(path).c_str(), 0)));