Make `Group` a type (#202)

This changes `Group` to be a type instead of a function, which means it
can support both grouping `[]Node` as well as doing variadic-ish slice things
like `Group{n1, n2}`.

This also means that `Map` can just return a `Group`.

Special thanks to @deitrix for this simple and elegant solution that,
for some reason or another, has had me perplexed for a long time.

Fixes #201.

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