Use more functions from the posterior package

[Aki Vehtari]

Release 2.7.0 imports posterior. We could use more posterior package functions. This would reduce possibility of having different implementations and thus reduce maintenance burden. Potential functions to replace

• ess_rfun
  • fft_next_good_size
  • autocovariance
• part of do_psis_i
  • psis_smooth_tail
  • n_pareto
  • enough_tail_samples
• ps_khat_threshold
• pareto_smooth
  • .pareto_smooth_tail
  • as_one_logical
  • should_return_NA
  • is_constant
• ess_tail
  • ess_quantile
  • .ess_quantile
  • .ess
  • .split_chains
• gpdfit
  • lx
  • adjust_k_wip
  • qgpd