feat: track_caller so panic reports call site, not lib

[dpezely]

• Have panic report user's call site (rather than the fn definition in lib.rs)
• Adds #[track_caller]
• Adds tests with optional manual instructions for visual confirmation
  • Tests mix std within the existing no_std module, albeit conditionally
  • Feel free to discard these tests since this is a mature Rust feature
• It's been available for rust libraries since 1.42 and generally available since Rust 1.46.0 released 9 Sep 2020
• See also: The track_caller attribute within Code generation attributes section of The Rust Reference