Type class for isomorphism checks

[ggazzi]

We should probably have a type class for types with isomorphism checks, similar to Eq. Something like:

class Iso a where
  (~=) :: a -> a -> Bool

We could also have a shorthand for not . (~=), which could be either (~/=) or (/~=).

This would help with issue #4.

[ggazzi]

I'd rather avoid adding the shorthand for not . (~=) initially, and only add it if we write a lot of code that would be improved by it.

[ggazzi]

Done at 4d3bb49