Another attempt to make SexpEqual functor consume a comparer function
but we cannot type check it, damn.
Move `length' function from applicative order Y imperative combinator
derivation to its dedicated home in `sexp-functions' folder.

Attempt to formalize our thoughts about imperative Y writing signatures
for it but it seems that we're tying ourself, so we do not constraint
    functors abscribing them to any signature.
Refactor `Y_bang' signature in order to not specify the type of the
initial value as functor argument but inlining it at `Y_bang' invocation,
as did with the parameter `L'. Now we have a clean signature and it
is possible to use the multi arguments version too.
Take apart the original file containing the fix point combinator
derivation in a set of small file.