src: add native URL class

Adds a URL native class for use within the node.js c/c++
code. This is primarily intended to be used by the eventual
ES6 modules implementation but can be used generally wherever
URL parsing within the c/c++ may be necessary.

```c
URL url1("http://example.org");
URL url2("foo", "http://example.org/bar");
URL url3("baz", &url2);
```

While we're at it, reduce reliance on macros to simplify impl.

Author: jasnell