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

Add zero-initialising versions of malloc{,Bytes,Array,Array0}

• Add calloc and callocBytes to Foreign.Marshal.Alloc.
• Add callocArray and callocArray0 to Foreign.Marshal.Array.

Proposal Discussion: https://www.haskell.org/pipermail/libraries/2014-November/024421.html