C functions for creating blocks from Haskell

[jspahrsummers]

The project's Objective-C component could expose a set of functions which create some standard block types (e.g., NSComparator, enumeration blocks, etc.) from a function pointer and some context. Then Haskell code can pass in function pointers and receive an opaque block (typed as an Id) in return.

typedef BOOL (*OHEnumerationFunction)(id, NSUInteger, HsStablePtr);
id OHEnumerationBlockWithFunction (OHEnumerationFunction, HsStablePtr);