#968: Add bytestring_to_uint16, uint32, uint64 functions

I can see the reverse() function for strings generally be useful but I
don't think it's a nice solution for converting endianess with
bytestring_to_count(). A default/optional argument would indeed be the
best approach I think. We've been kicking that idea around a few times
already, and I've now openened a ticket for that. For the time being,
I'd indeed go with the explicit endianess argument and then switch
over to an optional argument once we have that.