Update the records' description according to the new representation
Rewrite `predecessors` to not use :preceding

Haven't removed :preceding yet to not break all tests, but this is the
goal. The new representation will be just the adjacency list in :
adjacent.
Rewrite `transpose` for unweighted digraphs without :preceding
Make use of the already defined `weight` function
Unite the separate `in-edges` implementations

Too much code duplication.
Unite the `transpose` implementations reducing code duplication
Improve code layout in `in-edges`
Do not use :preceding in `add-edges'`
Do not use :preceding in `remove-edges'`
Do not use :preceding in `remove-all`
Comment out `remove-vertices'` for now

It is completely broken and should be rewritten.
Finally remove :preceding from the records' fields