* Implement support for variadic arguments to `m.fragment`
While I was at it, I refactored the common logic out of `hyperscript`.
* Add a missed change from #2326
* Update docs + changelog [skip ci]
* Explain rationale for `hyperscriptVnode`'s calling convention
This way, it doesn't get erroneously "cleaned up" into something worse,
and so it's clearer how it'd be potentially optimized once ES5 support
is dropped.
