This library almost disables multithreading while used with `RUST_BACKTRACE=1`

Hello!

I ran into a situation where computation-heavy code where errors occur often (just as a part of logic) has supressed multithread behavior by using roughly 1 CPU core instead of 48 available in the case `RUST_BACKTRACE=1`.

Of course, this code smells and heavy computations should not use such dynamic library, but I believe this behavior should be at least documented.