A bit more Split fettling in 04

A bit more Split fettling in 04

It was silly to have an operator that immediately called its operand on
its argument and did nothing else with the operand.  Better to have a
function that takes the output of the operand as its left argument.
That avoids the confusion I had over whether the operator should take a
function that acts on a single element or on the whole list: it now
takes neither.

This doesn't even make calling the function any harder. It just changes
F _split x into F⊸Split x.