termcolor: make StandardStream be Send
termcolor: make StandardStream be Send

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

BurntSushi/ripgrep,BurntSushi/ripgrep,BurntSushi/ripgrep,BurntSushi/ripgrep