Read a value from a memory area regarded as an array of values of the
same kind. The first argument specifies the start address of the array
and the second the index into the array (the first element of the
array has index
0). The following equality holds,
peekElemOff addr idx = IOExts.fixIO $ \result ->
peek (addr `plusPtr` (idx * sizeOf result))
Note that this is only a specification, not necessarily the concrete
implementation of the function.