Make backtrace support optional

I'm using anyhow for error management in an environment where I care about the binary size. Managing contexts explicitly lets me use rustc's panic=abort mode without significantly reducing ease of debugging, as the "stacktrace" is provided by anyhow. panic=abort would typically greatly reduce binary size because libunwind would no longer need to be linked in. However, anyhow still brings in std::backtrace, which relies on libunwind internally, reducing the benefits of panic=abort.

Would it be possible (or desired) to add an opt-in feature to avoid using std::backtrace, as if `RUST_BACKTRACE=0` was always set? Alternatively, what about making `backtrace` an opt-out feature?