Tracking issue for eRFC 2497, "if- and while-let-chains, take 2", edition changes

[Centril]

This is a tracking issue for the eRFC "if- and while-let-chains, take 2" (rust-lang/rfcs#2497) pertaining solely to the immediate Rust 2018 transition changes. For the main tracking issue, see #53667.

Steps:

• Implement the edition parts of the RFC (cc @rust-lang/compiler -- can anyone write up mentoring instructions?)
• Adjust documentation (see instructions on forge)
• Stabilization PR (see instructions on forge)

Unresolved questions:

There are none.

[Centril]

Nominating for discussion on whether we should try to do this for all editions or as exactly per the RFC (>=2018).

[nikomatsakis]

We discussed in the @rust-lang/lang meeting and decided that we would first try to do this for all editions, because the belief is that in practice this will not a problem — and in general it'd be nice to avoid bifurcating our grammar in this way if we can get away with it.

[Centril]

Triage:

• There isn't anything to stabilize here since we chose to implement this directly in Rust 2015+.
• The documentation still needs to be updated before closing this issue.