Alternative TailLoop

[cqc-alec]

See #215 .

It is unclear whether this exact formulation of TailLoop is the most
natural or useful. It may be that compilation typically uses multiple.
Another is:

A node with type `I -> O` with three children of types `A: I -> p + F`
(the output is the row formed by extending the row `F` with the boolean
output `p`), `B: F-> O` and `C: F -> I`. This node offers similar
benefits to the option above, exchanging the machinery of variants for
having a node with 3 children rather than 1. The semantics of the node
are:

1.  Execute A, outputting the boolean `p` and some outputs `F`

2.  If `p`, execute B with inputs `F` and return the output `O`

3.  Else execute `C` with inputs `F` and then restart loop with inputs
    `I`