New XML parser implementation

Custom XML parser implementation in pure JS that's orders of magnitude
faster than libxmljs mainly due to the overhead of JS-native calls in
libxmljs.

test/xml-parser.ts

@@ -0,0 +1,78 @@
+import ava from "ava";
+import {
+  parseXmlDeclaration,
+  decodeEntities,
+  parseXml
+} from "../lib/xml-parser";
+
+ava("parseXmlDeclaration", t => {
+  const buf = Buffer.from(
+    '<?xml version="1.0" encoding="UTF-8"?>\n<soap-env:Envelope />'
+  );
+  const attrs = parseXmlDeclaration(buf);
+  t.deepEqual(attrs, [
+    {
+      name: "version",
+      namespace: "",
+      localName: "version",
+      value: "1.0"
+    },
+    {
+      name: "encoding",
+      namespace: "",
+      localName: "encoding",
+      value: "UTF-8"
+    }
+  ]);
+});
+
+ava("decodeEntities", t => {
+  t.is(
+    decodeEntities("&&amp;&lt;&gt;&quot;&apos;&gt;&#167;&#xd842;&#xDFB7;;"),
+    "&&<>\"'>§𠮷;"
+  );
+});
+
+ava("parse", t => {
+  const xml =
+    '<?xml version="1.0"?>\n<a-b:c><d f="1<g>"/><!-- comment --><h >i</h></a-b:c>';
+  const parsed = parseXml(xml);
+  t.deepEqual(parsed, {
+    name: "root",
+    namespace: "",
+    localName: "root",
+    attrs: "",
+    text: "",
+    bodyIndex: 0,
+    children: [
+      {
+        name: "a-b:c",
+        namespace: "a-b",
+        localName: "c",
+        attrs: "",
+        text: "",
+        bodyIndex: 29,
+        children: [
+          {
+            name: "d",
+            namespace: "",
+            localName: "d",
+            attrs: 'f="1<g>"',
+            text: "",
+            bodyIndex: 42,
+            children: []
+          },
+          {
+            name: "h",
+            namespace: "",
+            localName: "h",
+            attrs: "",
+            text: "i",
+            bodyIndex: 62,
+            children: []
+          }
+        ]
+      }
+    ]
+  });
+});