Minor containers API changes

[Kazu Yamamoto ( 山本和彦 )]

Ivan,

>> Why the container package does not provide a type-class to unify APIs?
>> Are there any technical/historical reasons?
> 
> Various people have tried, but they're typically unweildy, and in some
> cases difficult to express without using extensions.  There's also
> tension between "this should be a class method" vs "this should be a
> function that uses the class" (large classes are annoying, but having
> a function as a class method allows for more potential
> implementation-specific optimisations).

Thank you for your explanation!

--Kazu