Auto merge of #64154 - alexcrichton:std-backtrace, r=sfackler

std: Add a `backtrace` module

This commit adds a `backtrace` module to the standard library, as
designed in [RFC 2504]. The `Backtrace` type is intentionally very
conservative, effectively only allowing capturing it and printing it.

Additionally this commit also adds a `backtrace` method to the `Error`
trait which defaults to returning `None`, as specified in [RFC 2504].
More information about the design here can be found in [RFC 2504] and in
the [tracking issue].

Implementation-wise this is all based on the `backtrace` crate and very
closely mirrors the `backtrace::Backtrace` type on crates.io. Otherwise
it's pretty standard in how it handles everything internally.

[RFC 2504]: https://github.com/rust-lang/rfcs/blob/master/text/2504-fix-error.md
[tracking issue]: https://github.com/rust-lang/rust/issues/53487

cc #53487

src/libstd/lib.rs

@@ -452,6 +452,7 @@ pub mod f64;
 #[macro_use]
 pub mod thread;
 pub mod ascii;
+pub mod backtrace;
 pub mod collections;
 pub mod env;
 pub mod error;