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

[Jim Apple]

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