hide panic internals in short backtraces

right now, nearly all foo.expect("bar") backtraces start like this:

stack backtrace:
   0: rust_begin_unwind
             at /rustc/7f2fc33da6633f5a764ddc263c769b6b2873d167/library/std/src/panicking.rs:652:5
   1: core::panicking::panic_fmt
             at /rustc/7f2fc33da6633f5a764ddc263c769b6b2873d167/library/core/src/panicking.rs:72:14
   2: core::panicking::panic_display
             at /rustc/7f2fc33da6633f5a764ddc263c769b6b2873d167/library/core/src/panicking.rs:256:5
   3: core::panicking::panic_str
             at /rustc/7f2fc33da6633f5a764ddc263c769b6b2873d167/library/core/src/panicking.rs:231:5
   4: core::option::expect_failed
             at /rustc/7f2fc33da6633f5a764ddc263c769b6b2873d167/library/core/src/option.rs:1994:5
   5: core::option::Option<T>::expect
             at /rustc/7f2fc33da6633f5a764ddc263c769b6b2873d167/library/core/src/option.rs:895:21

this is not super useful. it doesn't add any information, other than maybe the very last frame which says you called expect(). it would be nice to omit anything in the core::panicking module from the backtrace; and maybe expect_failed and the core::ops::function::FnOnce::call_once that show up at the end of the backtrace as well. people can always opt back in with RUST_BACKTRACE=full.

the code for this lives in https://github.com/rust-lang/rust/blob/378a43a06510f3e3a49c69c8de71745e6a884048/library/std/src/sys_common/backtrace.rs#L77-L81

@rustbot label +T-libs +A-runtime

Start at library/std/src/sys_common/backtrace.rs lines 77-81 and compare the current short backtrace with RUST_BACKTRACE=full. Define which core::panicking and trailing internal frames are omitted in short mode, while full mode continues to show them; done means expect() backtraces emphasize application frames.