Allow renaming RUST_BACKTRACE to MYCRATE_BACKTRACE

It would be nice if we could rename the RUST_BACKTRACE environment variable to something custom like MYCRATE_BACKTRACE. This idea came to me from looking at env_logger and how you can rename the environment variable it uses via from_env.

I suggest something along the lines of this:

#![backtrace_env = "MYCRATE_BACKTRACE"]

fn main() {
    // ...
}

And then enabling it via:

MYCRATE_BACKTRACE=1 ./mycrate

Supporting reasons:

• Gives some sense of consistency for end users who may not know what Rust is, where the current variable name may be confusing
• Some authors believe the end user has no business knowing what language their program was written in
• Backtraces may be enabled for some Rust programs but not all when executed in the same environment, such as in a shell script

Start by reviewing the RFC process and the existing discussion on issue #2599. A complete contribution would specify the proposed attribute, its environment-variable behavior, and acceptance criteria clearly enough for an RFC decision.