Support named "special" operations in WebIDL

This commit adds support for two different features of the "special" operations
in WebIDL. First, it implements the desugaring [described by WebIDL][1] where
this:

    interface Dictionary {
      getter double getProperty(DOMString propertyName);
      setter void setProperty(DOMString propertyName, double propertyValue);
    };

becomes ...

    interface Dictionary {
      double getProperty(DOMString propertyName);
      void setProperty(DOMString propertyName, double propertyValue);

      getter double (DOMString propertyName);
      setter void (DOMString propertyName, double propertyValue);
    };

where specifically a named `getter` generates both a getter and a named
function.

Second it implements the distinction between two different types of getters in
WebIDL, described as:

> Getters and setters come in two varieties: ones that take a DOMString as a
> property name, known as named property getters and named property setters, and
> ones that take an unsigned long as a property index, known as indexed property
> getters and indexed property setters.

The name `get` is given to DOMString arguments, and the name `get_idx` is given
to index property getters.

[1]: https://heycam.github.io/webidl/#idl-special-operations

crates/web-sys/build.rs

@@ -65,6 +65,7 @@ fn try_main() -> Result<(), failure::Error> {
         .context("writing bindings to output file")?;
 
     // run rustfmt on the generated file - really handy for debugging
+    println!("cargo:rerun-if-env-changed=WEBIDL_RUSTFMT_BINDINGS");
     if env::var("WEBIDL_RUSTFMT_BINDINGS").is_ok() {
         let status = Command::new("rustfmt")
             .arg(&out_file_path)