Print block slots somewhat helpfully

I noticed today that if my P/Invoke command line tool is instructed to print the value of a slot (readslot) that is a custom kind (i.e. defined with newslot block), it spews an error message that's almost certainly 100% incomprehensible and useless to most users. So today, I added a check for block slots in the readslot implementation to mention the length of the slot instead of attempting to marshal and stringify the value. The architecture is not currently set up such that the fields of a block can be printed; the information on each field's kind is not preserved, only the .NET Framework type, so it couldn't tell the difference between some arbitrary pointer-sized field and a string. Nevertheless, not spewing strange errors is an improvement.