Added some tvin-tests
Moved shared result field code into separate file
Removed _name_t from and added alias operators to result field.

A result field really is just a value, if you insist on using it as a
select column, give it a name via an alias.
Extracted result_field_t from value types.

This is a preparation for adding serializer specializations for them
Added missing result_field.h
Added some hints
Moved value types out of detail namespace
Adjusted usage of wrong_t.

I think there was a risk of wrong_t based static_asserts firing, if the
template parameters are used. Anyway, this is shorter.
Added value_or_null method/type