Generic writing interface to avoid boxing

[roji]

In ADO.NET, parameters are non-generic - you set values on DbParameter.Value which is an object. This is a weakly-typed API that forces value types to be boxed. Add an NpgsqlParameter class with TypedValue, for a strongly-typed API. See https://github.com/dotnet/corefx/issues/8955 for the same issue at the ADO.NET level.

Timing-wise, with the new type mapping API opening up for 3.3, this is the right moment to add support for this feature (since it will impact type handling APIs considerably).