in addition to /

as /walk is temporarily disabled for now

works for both random and non-random walks

Closes T2084

we need to fix T1969 before opening this up, and in most cases what one wants
is really /randomwalk anyway

Closes T1888

it is the only sane way to have decent semantics in presence of edge
restrictions, without having to visit twice all successors

Closes T2077

in wait for a migration to a more propre solution, i.e.,
https://docs.python.org/3/library/importlib.html#module-importlib.resources

it is a marginal dependency of LAW, for a fringe tool that we do not use

while at it, switch string formatting to f-strings

... and it is brittle enough to make the overall doc build fail fairly
often. We will reconsider building it when we can actually shipped it (see
T1971)