`Memory`: Remove bare `Monad m` constraints

[langston-barrett]

There are several functions in Data.Macaw.Memory that use a bare Monad m constraint. This is suspicious, because Monad alone doesn't expose any operations that would be useful to this code. If they use fail, they should depend on MonadFail, otherwise the constraint should be removed and these should be pure functions.

This would be a breaking change.

https://github.com/search?q=repo%3AGaloisInc%2Fmacaw%20path%3AMemory.hs%20%2FMonad%20m%2F&type=code

[sauclovian-g]

I would bet that if you look at the history they used to have more, and someone either didn't want to make the breaking change, didn't want to retype all the code as pure syntax, figured other monad bits would reappear later, or all of the above...