Remove type parameters from Store

[robin-nitrokey]

Depends on:

• Use DynFilesystem trait for FS operations #144

Alternatively, we could completely drop the Store trait and just use a struct with three references. To be tested whether that is more efficient.

[sosthene-nitrokey]

 Is this really necessary?

[robin-nitrokey]

It is not necessary, but I think it has multiple benefits: It leads to simpler code. It allows us to get rid of the unnecessary Fs type that is just another level of indirection and another static mut. And it allows us to be more flexible in Store implementations, especially e. g. for the virtual runners or when replacing a storage with RAM. So I would rather ask the question: Why do we need these type parameters?

[sosthene-nitrokey]

Good!