Mutable accessor for Copy single-element Cell-like container

[dtolnay]

The guideline says:

Single-element containers where the element is Copy (e.g. Cell-like containers) should instead return the value directly, and not implement a mutable accessor.

And yet: Cell::get_mut

[Rufflewind]

That guideline is quite confusing. Why does the presence of Copy make mutable accessors undesirable?

[jonas-schievink]

@Rufflewind My guess is that set should be used to put a new value into the container.

Cell::get_mut is no longer limited to Copy types, by the way, so it now matches the guideline.

[dtolnay]

Cell now works for non-Copy types and exposes get_mut for them, so I removed this special case from the guidelines in ee56222.