Patch: add ByteString.Unsafe.unsafePackMallocCStringLen

Hello. There's a minor lapse in Data.ByteString.Unsafe: unsafePackMallocCString, unlike the other unsafePack* -functions, doesn't have a variant with an explicit length. This is useful when interfacing with C libraries that return a blob of binary data that the caller must free. Attached is a darcs patch that adds it. Lauri