This opens path to per-reporter colour output customization,
and fixes multiple issues with the old colour implementation.
Under the old implementation, using Win32-backed colouring
would always change the colour used by the console, even if the
actual output was written elsewhere, such as a file passed by
the `--out` flag. This will no longer happen, as the reporter's
colour impl will check that the reporter's stream is pointed
to console before trying to change the colours.
POSIX/ANSI colour implementation suffered a similar-ish issue,
in that it only wrote the colour escape codes into the default
output stream, even if the reporter asking for colouring was
actually writing to a completely different output stream.
Also generalized the implementations to write to the provided
output stream, which will be required for the follow up changes,
where the listings should happen to the location user asked for
by specifying the `-o` flag.
Most of the changes are completely pointless renaming of constructor
arguments so that they do not use the same name as the type members,
but 🤷Closes#2015
Due to also adding a new TU, there is no improvement to the
compilation times of the static library, but it improves the
compilation times of consumer's reporter TUs.
