Write ASTs out as straw proposal text format

[fitzgen]

We can parse the straw proposal text format into an AST (see src/ast.rs and src/grammar.lalrpop).

We should also support taking an AST and writing it out into the straw proposal text format.

I'd define a trait like

trait Unparse {
    fn unparse<W>(&self, output: &mut W) -> std::io::Result<()>
        where W: std::io::Write;
}

and implement it for all of our AST types, and expose a top level helper utility that takes a whole ast::WebidlBindingsSection and recursively unparses the whole thing.