Experimental Tracking Issue for Heterogeneous Try Blocks

[scottmcm]

This is a tracking issue for experimental work related to rust-lang/rfcs#3721 (comment)
The feature gate for the issue is #![feature(try_blocks_heterogeneous)].

For homogeneous try blocks, see #31436

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?)
  • Experimentally add *heterogeneous* try blocks #149489
• 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
  • (non-blocking) Formatting has been implemented in rustfmt
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• try bikeshed: What type should be annotated? #154127
• try bikeshed: What should the syntax be? #154128

Implementation history

• Experimentally add *heterogeneous* try blocks #149489