This is essentially a rename of the existing `Stdout` type to `StandardStream`
and a change of its constructor from a single `new()` function to have two
`stdout()` and `stderr()` functions.
Under the hood, we add add internal IoStandardStream{,Lock} enums that allow
us to abstract between Stdout and Stderr conveniently. The rest of the needed
changes then fall out fairly naturally.
Fixes #324.
[breaking-change]