Allow expressions to have tuple-valued output type

[senderista]

There are cases where an expression must emit multiple columns which cannot be computed independently (i.e., by different expressions). One example (via @parmitam) is a chunking expression which explodes some input value into a sequence of chunks, with each chunk numbered by its sequence within the output of its input tuple: ["abc", "def"] -> [(1, "a"), (2, "b"), (3, "c"), (1, "d"), (2, "e"), (3, "f")]. It is impossible to assign these sequence numbers after the fact, since the association of the outputs with their inputs has been lost (as has, possibly, the original order of the outputs). This example could be solved by an expression that output tuples of type (LONG_TYPE, STRING_TYPE).

This will require extensive changes in Apply and related operators, since we can no longer associate emit expressions 1:1 with output columns.