Avoid move assignment for streams

[przemoc]

libstdc++ bundled with GCC older than 5 doesn't have movable streams. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53626

Using move assignment for stream can be easily avoided here. Also improve consistency and readability by not creating string for stream's filename explicitly.

[przemoc]

I'm removing my fork, as this is the only pull request not merged by @richox. PR will remain valid, though. https://docs.github.com/en/github/collaborating-with-issues-and-pull-requests/checking-out-pull-requests-locally

I'm just doing some 2021 spring cleanings. IMHO forks that are no longer source for fresh stuff (and all their feature branches were already converted into pull requests upstream) should be deleted, to not clutter the forkspace.

[przemoc]

Closing after 5 years.