While a bit primitive, it should get us at least a better number than
nothing.

The basic conversion is a straightforward conversion of the linear
recursion to a loop forwards and backwards propagation of the result.
But this uses an optimization to avoid the need for extra space that
would otherwise be necessary to store the stack of unfinished states as
the function is not tail recursive.

Observe that only non-root-nodes in cycles have a recursive call and
that every such call overwrites their own node state. Thus we reuse the
node state itself as temporary storage for the stack of unfinished
states by inverting the links to a chain back to the previous state
update. When we hit the root or end of the full explored chain we
propagate the node state update backwards by following the chain until
a node with a link to itself.

This allows constructing the sccc for large that visit many nodes before
finding a single cycle of sccc, for example lists. When used to find
dependencies in borrow checking the list case is what occurs in very
long functions.