Eliminate tuple udf

[willmurnane]

There's a lot of song and dance about how functions which return a single tuple are turned into something that Postgres can call. This eliminates that: when a function which returns a tuple is discovered, the func_call is rewritten into an iterator on the spot, and then it's treated as if it were a function which returns an iterator (and thus passes through the impl_table_srf function). This removes:

• the impl_tuple_udf function
• the tuple return type of item_fn
• the edge case of "this might not need a pg_guard"

[workingjubilee]

This will conflict with #615, which may still yet receive final-pass revisions. It cannot land before then, and after #615 does, I will be pushing towards 0.5.0.

[willmurnane]

Closing in favor of #685. Now that #615 has landed, all of these functions are unused and can be removed.