allow Add to *replace* an existing template parameter in DynamicParameters, allowing for out/return/etc usage

fix 'could destabilise runtime' error if structs threw an error; also tidied up the locals and made the func return the right type

added support for schema changes - schema can now change and dapper will re-generate the deserializers.

added support for schema changes - schema can now change and dapper will re-generate the deserializers.

EG:

  DynamicParameters p = new DynamicParameters();
  p.AddDynamicParams(new { A = 1, B = 2 });
  p.AddDynamicParams(new { C = 3, D = 4 });

Expose some more of dapper's internals