HashType: Rename to HashAlgorithm

To be consistent with CLI, nix API
and many other references.

As part of this, we also converted it to a scoped enum.

https://github.com/NixOS/nix/issues/8876

tests/unit/libstore/path-info.cc

@@ -25,7 +25,7 @@ static UnkeyedValidPathInfo makePathInfo(const Store & store, bool includeImpure
         "foo",
         FixedOutputInfo {
             .method = FileIngestionMethod::Recursive,
-            .hash = hashString(HashType::htSHA256, "(...)"),
+            .hash = hashString(HashAlgorithm::SHA256, "(...)"),
 
             .references = {
                 .others = {