Making the `mut` keyword less misleading

[doublequartz]

Ideally, something like this would be implemented:

@steffahn

If let mut vs. let is supposed to be distinguished by the guarantee that the latter cannot be mutated between the initialization of the variable and any of its use cases (ignoring interior mutability of course), then I think there is not really any reason against allowing a single, final mutable access of the variable.

More realistically, we just continue to allow moves unless they're behind a physical & reference.