This commit fixes a bug where the `StandardStream` type isn't `Send` on Windows. This can cause some surprising compile breakage, and the only motivation for it being non-Send was dubious. Namely, it was a result of trying to eliminate code duplication. This refactoring also eliminates at least one "unreachable" panic case that was a result of trying to eliminate code reuse, so that's a nice benefit as well. Fixes #503