This can be used from rdbg from example, to generate session files.

In particular, 'rdbg -lurette' now works thanls to this option
(cf rdbg sha 63566a893807361691137cafe15854a2a7395eec)

Remove the --generate-rdbg-load one that is less general

This is necessary because I cannot Dynlink .cmxs files from a ocaml
toplevel. Hence, one way to load the algos from rdbg is to
#load .cma files. This option generates an ocaml file that
can be #used by automatically generated rdbg-session.ml files.

For some reason, I believed that the source dir containing the main
program need to be named bin for dune to work smoothly. But actually,
it seems to work fine with src, so let's go for it.

This is necessary to be able to use sasa with luciole.  Indeed, in
rdbg/lurette, luciole always plays first. Hence if we want sasa to
play after luciole, we can use the option.