Remove HashType::Unknown

Instead, `Hash` uses `std::optional<HashType>`. In the future, we may
also make `Hash` itself require a known hash type, encoraging people to
use `std::optional<Hash>` instead.

src/libutil/tests/hash.cc

@@ -75,6 +75,6 @@ namespace nix {
 
     TEST(hashString, hashingWithUnknownAlgoExits) {
         auto s = "unknown";
-        ASSERT_DEATH(hashString(HashType::Unknown, s), "");
+        ASSERT_DEATH(hashString(HashType::SHA512, s), "");
     }
 }