Add RefCell<T>::replace_with

[Havvy]

I also moved the Panic sections to before examples in the other two functions also under this feature gate, and changed the variable names in replace to be more readable.

r? @rust-libs

[kennytm]

(cc @rust-lang/libs)

[Havvy]

r? @BurntSushi

Just to get some reviewer on this.

[shepmaster]

Ping from triage, @BurntSushi — will you be able to spend some time on this?

[aturon]

@bors: r+

Thanks!

[bors]

📌 Commit 2d02772 has been approved by aturon

[bors]

⌛ Testing commit 2d02772 with merge 5802986...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: aturon
Pushing 5802986 to master...

[RalfJung]

In which sense does RefCell::replace_with correspond to std::mem::replace? It seems to me that part of the doc was accidentally copied?

[Havvy]

In the same sense that RefCell::replace corresponds to mem::replace. Though with the with part, it doesn't correspond exactly. It could be worded more explicitly "Like RefCell::replace, this function corresponds to mem::replace."

[RalfJung]

Well, RefCell::replace and RefCell::replace_with differ, so if mem::replace corresponds to the former, it doesn't correspond to the later. ;)