Add round-tripping tests for Instant.DateTime
Add round-tripping tests for Instant.DateTime

And remove false statement about DateTime being the only means of
creating an arbitrary Instant. There's also .from-posix().

bitrauser/roast,dogbert17/roast,dankogai/roast,zostay/roast,skids/roast,dankogai/roast,b2gills/roast,b2gills/roast,dankogai/roast,b2gills/roast,perl6/roast,dogbert17/roast,zostay/roast,zostay/roast,skids/roast,bitrauser/roast,skids/roast