Rewriting this with `volatile` crate?

[paulsohn]

Although I'm adding features of accessor crate now,
I noticed that my concept is similar to that of existing volatile crate.

e.g. introducing new bound accessor types over unbounded accessors are somewhat tiresome,
so we make a generic lifetime parameter on accessor types (and set default lifetime to 'static) etc..

The thing that lacks in volatile ptr is that there are no mappers, however.

(I should go to work shortly so I will add details more tomorrow)