Modify unique_everseen to be fast for mostly-hashable sequences.
Modify unique_everseen to be fast for mostly-hashable sequences.

- Instead of switching from a set to a list on the first non-hashable value,
  keep both a set and a list around in parallel. This is slightly slower
  when the sequence is mostly non-hashable, but much faster when it's mostly
  hashable.

erikrose/more-itertools,more-itertools/more-itertools