make std.event.Loop work with --single-threaded

[andrewrk]

See #1764 (comment)

On MacOS and FreeBSD it will be worth looking into whether kqueue works with file system operations.

When #1907 is implemented, it could be interesting to explore io_submit on Linux instead of epoll, however even so, we would need another implementation for older linuxes that does not rely on a thread.

This issue could potentially require a proposal to modify the single-threaded flag to work at some kind of scope level. Or it could potentially be resolved by making async file system functions actually be blocking in --single-threaded mode.