* Generalize `AsIter` to allow iterating over contents of a `RefCell`
Towards `MapObserver`s in safe Rust.
* Helpers for `RefCellValueObserver`
* MapObserver: Return owned Self::Entry from .get()
`Self::Entry` is `Copy`, so there's not much value in returning a
reference from `get()`. Futhermore, returning a reference limits the
possible implementations of `MapObserver`, because it forces the
borrow/reset to outlive the body of the method.
* MapObserver: Replace `.get_mut()` with `.set(idx, val)`
Like the previous commit, this is intended to expand the possible
implementations of `MapObserver` to types with interior mutability,
which can't necessarily loan out their content.
* Make `RefCellValueObserver` into a safe `MapObserver`
* as iter mut
* as slice (mut): allow for non-& refs
* nostd
* CI round 1
* cleanup + AsSlice defs for RefCellValueObserver
* clippy fixes
* avoid unnecessary imports
* whoops, too aggressive
* use deref instead of as slice
* whoops
* fix as slice conditional importing in stable
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Co-authored-by: Addison Crump <addison.crump@cispa.de>
b0248461e2