Tracking Issue for homogeneous `try_blocks`

[scottmcm]

This is a tracking issue for the RFC "Homogeneous Try Blocks" (rust-lang/rfcs#3721).
The feature gate for the issue is #![feature(try_blocks)].

This obviates the previous #31436 tracking issue (which had included other things like the ? operator itself and has gotten huge).

Note that this does not include heterogeneous try blocks; for those see #149488.

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.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Implement the RFC (cc @rust-lang/XXX -- can anyone write up mentoring
  instructions?)
  • Implement the alternative try block desugaring #148725
• Adjust documentation (see instructions on rustc-dev-guide)
• Style updates for any new syntax (nightly-style-procedure)
  • Style team decision on new formatting
  • Formatting for new syntax has been added to the Style Guide
    • style-guide: Add try blocks #152311
  • (non-blocking) Formatting has been implemented in rustfmt
    • Consider try blocks as block-like for overflowed expr #153445
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• How should the trait be named and structured?
  • Moved to Tracking Issue for ops::Residual (feature try_trait_v2_residual) #91285 as it's arguably not part of the lang tracking issue.

Implementation history

• Implement the alternative try block desugaring #148725
• Update try_blocks to a new tracking issue number #154409