Improve view design for efficiency

[ptal]

The current design just copy the data of the variable store and apply the view function on the data copied. However if the user just want to call contains(b) on the variable, it might be more efficient to apply the view function to b.

To achieve a clean design, I think we need to parametrize associated type of read with lifetimes. So this is postponed until this feature land in the rust compiler.

[ptal]

This is a useless optimization, much better stuff remain to be done before going at this level of details.