std: Add a backtrace module

[alexcrichton]

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.

cc #53487