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/fragment.md

@@ -37,9 +37,9 @@ Generates a fragment [vnode](vnodes.md)
 
 Argument    | Type                                                | Required | Description
 ----------- | --------------------------------------------------- | -------- | ---
-`attrs`     | `Object`                                            | Yes      | A map of attributes
-`children`  | `Array<Vnode|String|Number|Boolean|null|undefined>` | Yes      | A list of vnodes
-**returns** | `Vnode`                                             |          | A fragment [vnode](vnodes.md)
+`attrs`     | `Object`                                            | No       | HTML attributes or element properties
+`children`  | `Array<Vnode>|String|Number|Boolean`                | No       | Child [vnodes](vnodes.md#structure). Can be written as [splat arguments](signatures.md#splats)
+**returns** | `Vnode`                                             |          | A fragment [vnode](vnodes.md#structure)
 
 [How to read signatures](signatures.md)
 
@@ -49,14 +49,14 @@ Argument    | Type                                                | Required | D
 
 `m.fragment()` creates a [fragment vnode](vnodes.md) with attributes. It is meant for advanced use cases involving [keys](keys.md) or [lifecyle methods](lifecycle-methods.md).
 
-A fragment vnode represents a list of DOM elements. If you want a regular element vnode that represents only one DOM element, you should use [`m()`](hyperscript.md) instead.
+A fragment vnode represents a list of DOM elements. If you want a regular element vnode that represents only one DOM element and don't require keyed logic, you should use [`m()`](hyperscript.md) instead.
 
-Normally you can use simple arrays instead to denote a list of nodes:
+Normally you can use simple arrays or splats instead to denote a list of nodes:
 
 ```javascript
 var groupVisible = true
 
-m("ul", [
+m("ul",
 	m("li", "child 1"),
 	m("li", "child 2"),
 	groupVisible ? [
@@ -64,7 +64,7 @@ m("ul", [
 		m("li", "child 3"),
 		m("li", "child 4"),
 	] : null
-])
+)
 ```
 
 However, Javascript arrays cannot be keyed or hold lifecycle methods. One option would be to create a wrapper element to host the key or lifecycle method, but sometimes it is not desirable to have an extra element (for example in complex table structures). In those cases, a fragment vnode can be used instead.