Add 'subsequences' and 'permutations' to Data.List (ticket #1990)

Hello all, As Thomas Hartman noted in a recent cafe thread [1], haskell 1.3 included the functions 'subsequences' and 'permutations'. I think these functions are quite useful, and I don't know why they were ever removed. This is a proposal to add these two functions to Data.List. The implementation is taken directly from the Haskell 1.3 report [2]. Trac ticket: #1990 Discussion period ends: January 7th (since there is a holiday comming up) Twan [1] http://article.gmane.org/gmane.comp.lang.haskell.cafe/33535 [2] http://haskell.cs.yale.edu/haskell-report/List.html