Rollup merge of #123356 - joboet:set_current_size, r=ChrisDenton

Reduce code size of `thread::set_current`

#123265 introduced a rather large binary size regression, because it added an `unwrap()` call on a `Result<(), Thread>`, which in turn pulled its rather heavy `Debug` implementation. This PR fixes this by readding the `rtassert!` that was removed.

Author: matthiaskrgr