Add 'subsequences' and 'permutations' to Data.List
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 (http://haskell.cs.yale.edu/haskell-report/List.html).