Help optimize out backtraces when disabled
The comment in `rust_backtrace_env` says:
> // If the `backtrace` feature of this crate isn't enabled quickly return
> // `None` so this can be constant propagated all over the place to turn
> // optimize away callers.
but this optimization has regressed, because the only caller of this function had an alternative path that unconditionally (and pointlessly) asked for a full backtrace, so the disabled state couldn't propagate.
I've added a getter for the full format that respects the feature flag, so that the caller will now be able to really optimize out the disabled backtrace path. I've also made `rust_backtrace_env` trivially inlineable when backtraces are disabled.
1cb57e2d2b