Allow variadic arguments to m.fragment (#2328)

* 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.

docs/change-log.md

@@ -54,6 +54,7 @@
     - All of the `m.*` properties from `mithril` are re-exported as named exports in addition to being attached to `m`.
     - `m()` itself from `mithril` is exported as the default export.
     - `mithril/stream`'s primary export is exported as the default export.
+- fragments: allow same attrs/children overloading logic as hyperscript ([#2328](https://github.com/MithrilJS/mithril.js/pull/2328))
 
 #### Bug fixes