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
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