Proposal: Add function to create new hash tables with a default size

In other some standard libraries (Java, OCaml), hash tables can be created with a user-supplied size. This avoids some resizing for users who know they will be inserting a lot of data. Data.HashTable does not expose a function to do this. This proposal changes that. For a dictionary of 5 million strings, this patch saves me about 33% of the total execution time. For 10 million strings, this patch saves me 50% of the total execution time. http://hackage.haskell.org/trac/ghc/ticket/4193