Support for ToTokens in phf_codegen for interop with quote crate

[krh]

I'm using quote to generate code and I have to go through:

        let map: TokenStream = self.map.build().to_string().parse().unwrap();                                                                                                                                         

to get a TokenStream - would it be useful to provide a ToTokens impl for phf_codegen::DisplayMap and friends?