Optional checks to make 'poke' safer

[mgsloan]

As documented on the size method, if size reports a different byte size than is actually used by poke, memory corruption, segfaults, and/or PokeExceptions may occur. This is freaky! It's done in the name of performance, in order to avoid checking it all the time.

One approach to this is to provide a lot of ways to safely define Store instances. The good news is that current store is pretty good for this. You just need to:

1. Use the well-tested instanced provided by the library

2. Use Generics / TH for deriving instances for your data types

3. Use the testing utilities on any of your custom instances (and perhaps the derived ones too)

The need for (3) could potentially be reduced by implementing #41