[feature request] Phantom node (or fragment to be the same as react)

[richard-uk1]

When I'm using this library, sometimes I have problems flattening all my data into a bumpalo::*::Vec to use as the children for a node. react solves this problem with fragments, a special node type that, rather than rendering an element in the dom, transparently inserts its children into the parent element at the right point.

It might make the library faster as well, since there will be less accidental re-allocations from use of bumpalo::*::Vec::push.

[fitzgen]

If you want to work on a PR, I'm happy to review and help out, but I don't have the cycles to do this myself.