Tracking Issue for RFC 3216: Allow using `for<'a>` syntax when declaring closures

[pnkfelix]

This is a tracking issue for the RFC "Allow using for<'a> syntax when declaring closures" (rust-lang/rfcs# 3216).
The feature gate for the issue is #![feature(- Feature Name: closure_lifetime_binder)].

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Implement the RFC (cc @rust-lang/XXX -- can anyone write up mentoring
  instructions?)
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

XXX --- list all the "unresolved questions" found in the RFC to ensure they are
not forgotten

Implementation history