Implement `calloc{,Bytes,Array,Array0}` allocators

Hi everyone, There is a proposal [1] (with implemented revision) to implement zero-initialising versions of malloc{,Bytes,Array,Array0} - Add calloc and callocBytes to Foreign.Marshal.Alloc. - Add callocArray and callocArray0 to Foreign.Marshal.Array. The benefit is that you can allocate required amount of memory and be certain that it will be cleared (set with zeroes) before you use it. Original discussion took a place in `fillBytes` [1] and was publicised on that mailing list [2]. Please share your thoughts. Thank you. [1] https://phabricator.haskell.org/D527 [2] https://phabricator.haskell.org/D461#11541 [3] https://www.haskell.org/pipermail/libraries/2014-November/024251.html -- alex p