Support From generation for Backtrace in stable rust

[mikea]

Currently Backtrace support has a (documented) limitation of requiring nightly rust. Presumably, this is to use provide API.

We'd like to use Backtrace and #[from] generation in stable rust, but it currently is tied to unstable error_generic_member_access.

Backtrace is stable since rust 1.62.0 would be nice to use it without implementing From manually and use provide api only when it is available.

See #387 for more discussion

[mikea]

BTW I am willing to work on a PR to fix this, but I'd like to get some approval to fix it before I go out of my way.

[WhyNotHugo]

I think a feature flag to disable nightly-dependant features would be ideal here.