Modernize L.OSM.XMLParser implementation

[mmd-osm]

Follow up for:

Even better would be to refactor the XML parser along the same lines as the JSON parser after fixing the XML tests and before adding the JSON parser but that's obviously not essential so I wouldn't insist on that.

Here's an attempt to make the XML parser resemble the JSON implementation. All the getElementsByTagName and getAttribute tend to make the code more difficult to read, compared to the JSON version. YMMV.

[tomhughes]

Given the issues using map that @AntonKhorev identified I think this is probably good to merge as is, thanks.