Add custom load npz function

[mthrok]

When loading data from NPZ format downloaded from remote source, np.load function can be bottleneck. This PR adds spdl.io.load_npz function, which loads NPZ file faster than the official implementation.

TODO:

• Bundle zlib into binary distribution.
• Explore the replacement of np.lib.format.read_array function
  Loading using np.fromfile seems to be faster, but the current approach won't trigger this branch, as SPDL's custom ZipFile class does not detected as file-like object by isfileobj function. Re-implementing np.fromfile might be more straightforward, (but comes with more maintenance cost.)