no_std backtrace

[roblabla]

I'd like to use failure in no_std with backtraces. Trouble is, failure doesn't disable the default features of backtrace, which means the "std" feature of backtrace is enabled.

IMO, it should be up to the final user (the binary crate) to determine the used features of backtrace-rs. Most of it is configuration of which library to use to unwind and symbolicate.

[leo60228]

A similar problem is the other way around; I'd like to use std without backtraces.