Tomap() creates a lua table with both names and number indexes.
Each named field stored by its name AND by its index in a tuple.
For example, if a tuple is {'a', 'b', 'c'} and its format is
{'field1', 'field2'}, then t.field1 is the same as t[1], t.field2 is
the same as t[2].

Not named fields can be accessed only by their indexes. For the example
above 'c' can be accessed only as t[3].

Closes #2821