Add std.parseUrl

Having a `std.parseUrl(str)` that returns a JSON object with "scheme", "host", "path", "query" and "fragment" fields would be useful for generating configurations where some applications accept parts of the URL as individual fields.

I suppose "host" could be more complicated, e.g. it may be useful to have "host" and "port" separated out, it may be useful to have "hostport" as a single string, have "port" always populated with the default for the "scheme", etc.