bug#27560: 26.0.50; seq-uniq is slow

[Nicolas Petton <nicolas@petton.fr>]

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Oleh (in Cc) did some benchmarks in May 2015, and `seq-uniq' is quite
slow:

    (setq cands (locate-file-completion-table
		 load-path (get-load-suffixes) "" nil t))
    (length cands)
    5357
    (length (cl-remove-duplicates cands :test 'equal))
    2481
    (benchmark-run (cl-remove-duplicates cands :test 'equal))
    (0.67873101 0 0.0)
    (benchmark-run (helm-fast-remove-dups cands :test 'equal))
    (0.001350054 0 0.0)
    (benchmark-run (seq-uniq cands 'equal))
    (5.270219822 27 2.396615401000002)

One easy way to make it faster would be to use `cl-remove-duplicates'
for sequences (sequencep), and default to the current implementation for
other seqp data structures (which means stream.el currently AFAIK).

Nico

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