Tracking issue for rust-lang/rfcs#243 and rust-lang/rfcs#1859.

Implementation concerns:

• ? operator that is roughly equivalent to try! - implement the ? operator #31954
• try { ... } expression - implement catch expressions #39849
  • resolve do catch { ... } syntax question
    • Resolved as try { .. }, - Tracking issue for RFC 2388, reserve the try keyword and resolve do catch { .. } syntax question with try { .. } #50412
  • resolve whether catch blocks should "wrap" result value (first addressed in catch blocks are not Ok-wrapping their value #41414, now being settled anew in Resolving Ok-wrapping for try blocks #70941)
  • Add a test confirming that it's an ExprWithBlock, so works in a match arm without a comma
  • Address issues with type inference (try { expr? }? currently requires an explicit type annotation somewhere).
• settle design of the Try trait (extend ? to operate over other types rfcs#1859)
  • implement new Try trait (in place of Carrier) and convert ? to use it (Lower ? to Try instead of Carrier #42275)
  • add impls for Option and so forth, and a suitable family of tests (Impl Try for Option #42526)
  • improve error messages as described in the RFC (Improve ? error messages per RFC 1859 #35946)
• reserve try in new edition
• block try{}catch (or other following idents) to leave design space open for the future, and point people to how to do what they want with match instead