Support base-64 hashes

perl/lib/Nix/Store.xs

src/libexpr/primops.cc

src/libstore/binary-cache-store.cc

src/libstore/build.cc

src/libstore/derivations.cc

src/libstore/download.cc

src/libstore/export-import.cc

src/libstore/gc.cc

src/libstore/local-store.cc

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

src/libstore/nar-info.cc

src/libstore/optimise-store.cc

src/libstore/remote-store.cc

src/libstore/store-api.cc

src/libutil/hash.cc

src/libutil/hash.hh

src/nix-daemon/nix-daemon.cc

src/nix-prefetch-url/nix-prefetch-url.cc

src/nix-store/nix-store.cc

src/nix/hash.cc