WIP: Make Hash always store a valid hash type

perl/lib/Nix/Store.xs

src/libexpr/primops.cc

src/libexpr/primops/fetchTree.cc

src/libfetchers/tree-info.cc

src/libfetchers/tree-info.hh

src/libstore/binary-cache-store.cc

src/libstore/build.cc

src/libstore/builtins/fetchurl.cc

src/libstore/daemon.cc

src/libstore/derivations.cc

src/libstore/export-import.cc

src/libstore/legacy-ssh-store.cc

src/libstore/local-store.cc

src/libstore/nar-info-disk-cache.cc

src/libstore/nar-info.cc

src/libstore/nar-info.hh

src/libstore/references.cc

src/libstore/references.hh

src/libstore/remote-store.cc

src/libstore/store-api.cc