Update println! calls to dbg! where appropriate

[mikemorris]

Started working on this locally on an older checkout and then discovered it was mostly added in 5813399 so rebased to resolve the conflict with the newer explanation, but can still help finish updating the later references to close #1658

Initially searched for matches on {: to target pretty-prints, then against plain println!("{}", calls that could be included too. Some remaining cases may exist but currently print additional context or could be a little more ambiguous. Ignored usage prior to introducing dbg! in chapter 5.

Checked and decided not to update:

• listings/ch06-enums-and-pattern-matching
  • https://github.com/rust-lang/book/blob/main/listings/ch06-enums-and-pattern-matching/no-listing-09-variable-in-pattern/src/main.rs#L22
  • https://github.com/rust-lang/book/blob/main/listings/ch06-enums-and-pattern-matching/no-listing-14-count-and-announce-if-let-else/src/main.rs#L20
  • println! calls appear to be part of the core program logic, not just debug output
• listings/ch09-error-handling
  • https://github.com/rust-lang/book/blob/main/listings/ch09-error-handling/listing-09-04/src/main.rs#L8
  • https://github.com/rust-lang/book/blob/main/listings/ch09-error-handling/listing-09-05/src/main.rs#L12
  • https://github.com/rust-lang/book/blob/main/listings/ch09-error-handling/no-listing-03-closures/src/main.rs#L8
  • panic and pretty-print of errors is program logic, not debug output
• listings/ch15-smart-pointers
  • https://github.com/rust-lang/book/blob/main/listings/ch15-smart-pointers/listing-15-24/src/main.rs#L21-L23
  • https://github.com/rust-lang/book/blob/main/listings/ch15-smart-pointers/listing-15-26/src/main.rs#L25
  • https://github.com/rust-lang/book/blob/main/listings/ch15-smart-pointers/listing-15-28/src/main.rs#L21
  • https://github.com/rust-lang/book/blob/main/listings/ch15-smart-pointers/listing-15-29/src/main.rs#L47
  • context around ordering, leaf parent, next item feels relevant
• listings/ch16-fearless-concurrency
  • https://github.com/rust-lang/book/blob/main/listings/ch16-fearless-concurrency/listing-16-03/src/main.rs#L7
  • https://github.com/rust-lang/book/blob/main/listings/ch16-fearless-concurrency/listing-16-04/src/main.rs#L7
  • https://github.com/rust-lang/book/blob/main/listings/ch16-fearless-concurrency/listing-16-05/src/main.rs#L7
  • would change program logic to no longer be invalid inside closure
• listsings/ch18-patterns-and-matching
  • https://github.com/rust-lang/book/blob/main/listings/ch18-patterns-and-matching/listing-18-27/src/main.rs
  • https://github.com/rust-lang/book/blob/main/listings/ch18-patterns-and-matching/listing-18-11/src/main.rs
  • println! calls appear to be part of the core program logic, not just debug output

[nyurik]

I did some println changes in #3048, and some might conflict. Not sure which should be used where... I guess anything that doesn't print additional information should use dbg!()...?