Unnecessary `&mut V` with `get_or_set_with` and `try_get_or_set_with` (CachedAsync)

[hanako-eo]

Hi, I'm using the library for a project and I notice that get_or_set_with and try_get_or_set_with return a &mut V and not a &V.
I wonder why you do it this way (because, according to the course, it's not possible to have 2 get_or_set values at the same time without having problems with the compiler). In my case, I need a &V and I would find it more relevant to prefix _mut and then return &mut V.