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.

render/hyperscriptVnode.js

@@ -0,0 +1,53 @@
+"use strict"
+
+var Vnode = require("../render/vnode")
+
+// Call via `hyperscriptVnode.apply(startOffset, arguments)`
+//
+// The reason I do it this way, forwarding the arguments and passing the start
+// offset in `this`, is so I don't have to create a temporary array in a
+// performance-critical path.
+//
+// In native ES6, I'd instead add a final `...args` parameter to the
+// `hyperscript` and `fragment` factories and define this as
+// `hyperscriptVnode(...args)`, since modern engines do optimize that away. But
+// ES5 (what Mithril requires thanks to IE support) doesn't give me that luxury,
+// and engines aren't nearly intelligent enough to do either of these:
+//
+// 1. Elide the allocation for `[].slice.call(arguments, 1)` when it's passed to
+//    another function only to be indexed.
+// 2. Elide an `arguments` allocation when it's passed to any function other
+//    than `Function.prototype.apply` or `Reflect.apply`.
+//
+// In ES6, it'd probably look closer to this (I'd need to profile it, though):
+// module.exports = function(attrs, ...children) {
+//     if (attrs == null || typeof attrs === "object" && attrs.tag == null && !Array.isArray(attrs)) {
+//         if (children.length === 1 && Array.isArray(children[0])) children = children[0]
+//     } else {
+//         children = children.length === 0 && Array.isArray(attrs) ? attrs : [attrs, ...children]
+//         attrs = undefined
+//     }
+//
+//     if (attrs == null) attrs = {}
+//     return Vnode("", attrs.key, attrs, children)
+// }
+module.exports = function() {
+	var attrs = arguments[this], start = this + 1, children
+
+	if (attrs == null) {
+		attrs = {}
+	} else if (typeof attrs !== "object" || attrs.tag != null || Array.isArray(attrs)) {
+		attrs = {}
+		start = this
+	}
+
+	if (arguments.length === start + 1) {
+		children = arguments[start]
+		if (!Array.isArray(children)) children = [children]
+	} else {
+		children = []
+		while (start < arguments.length) children.push(arguments[start++])
+	}
+
+	return Vnode("", attrs.key, attrs, children)
+}