Marker type for quote_name()

[jtv]

Similar to what I've done with pqxx::prepped, it'd be nice to have a marker type to indicate the need to quote a string as an identifier (e.g. a table or column name). We can then teach quote() to deal with that specially.

That could be two steps on the way to making quote() universal: "quote this value if & as needed give its type." Once we have that, it might become feasible to do generic client-side insertion of parameter values — which we could then eventually use to implement parameters on streaming queries.