WIP: Add a module representing memory

[bechang]

@bennostein @kyleheadley One more thing I wanted to get in from an interface standpoint is the Mem module here to show you how we can enforce the invariant that a (concrete) address is allocated internally and cannot be fabricated (while still doing it reasonably in a functional manner).

This is a essentially port of something I have in the 3155 class, except I pulled in the State monad from cats rather than my homegrown one for the class.

This is probably not ready to merge because there are no tests or anything to use it. But I want to have it at least here for you to consider.

[kyleheadley]

The code looks simple enough, but I don't have knowledge of how this would be used. If it's standard, I say merge and we can edit later. If not, lets get some tests up so we can see if it's what we want.

[bechang]

I guess this is back in my court. There should be some tests to show it is supposed to work. Thanks, @kyleheadley.