add support for deserializing and printing `std::backtrace::Backtrace`

[yaahc]

The nightly std::backtrace::Backtrace has a debug format that is relatively easy to parse. There are plans to eventually add a frame iterator api to backtrace similar to backtrace-rs's Backtrace but it seems rather far off. Until then I think it makes sense to write a deserializer for the Debug format of backtrace, which should hopefully be pretty stable, and use that to build the Frame objects used internally to print the backtrace.

[bjorn3]

which should hopefully be pretty stable

Debug implementations are not stable. Don't parse it! rust-lang/rust#59076 caused a lot of test breakage, because people depended on the exact debug formatting of structs in pretty print mode.

cc rust-lang/rust#62794.

[ArekPiekarz]

It looks like it's a duplicate of #24. But it means that more people care about solving this problem.

[yaahc]

aah, fair enough, ill close this and add my comments to the other issue.