`Fetcher` api for multiple mutable value access.

[rhedgeco]

This is intended as an initial discussion PR and not a completed/documented feature. While the feature will work as submitted. I believe more discussion is required, and an obvious clean up would be due.

I am often burdened by the lack of aliasing hints for the rust compiler and end up wanting multiple mutable access to data that should be completely safe. Since there is no way to tell a method what it aliases, it is automatically assumed that it aliases over the entire struct that it is attached to.

Because of the above, it is impossible to get multiple mutable values out of a map. This PR is an attempt to create a discussion around a potential solution to that problem.

Please let me know what you think.