Remove most uses of ColouredString::slice().

Remove most uses of ColouredString::slice().

The new flexibility means we don't have to use an explicit method call
to turn a ColouredString into a ColouredStringSlice before passing it
to ColouredString::push_str().

Also, in tui.rs, ratatui_set_string demonstrates that functions
outside the coloured_string module can also be given the flexibility
to take either an owned ColouredString or a slice, by taking an
argument 'impl ColouredStringCommon'.

However, in FileStatusLineFinal::render_highlighted(), where I pass
ColouredStringSlices to a closure, I _couldn't_ do that - closures are
apparently more restricted than ordinary functions in that way. So
that's the one place where I still had to explicitly call .slice().