Support greater than 63 orbitals

[kevinsung]

ffsim currently cannot support more than 63 orbitals, even with only one or two electrons, due to the way PySCF represents electronic configurations with integers. For example:

import ffsim

vec = ffsim.slater_determinant(64, [0])

Traceback (most recent call last):
  File "/home/kevinsung/projects/ffsim/scratch/large_norb.py", line 3, in <module>
    vec = ffsim.slater_determinant(64, [0])
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/kevinsung/projects/ffsim/python/ffsim/states/slater.py", line 96, in slater_determinant
    alpha_index = cistring.str2addr(norb, n_alpha, alpha_string)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/kevinsung/.pyenv/versions/ffsim/lib/python3.12/site-packages/pyscf/fci/cistring.py", line 385, in str2addr
    raise NotImplementedError('norb >= 64')
NotImplementedError: norb >= 64

This would likely be a significant undertaking.