Bump sexplib0

[patricoferris]

Our OCaml lower-bound is already 4.08.

With newer versions of dune, we are getting the following warning.

File "dune-project", line 21, characters 2-42:
21 |   (sexplib0 (and :with-test (>= "v0.15"))) ; Printexc.register_printer in sexplib0 changed
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Warning: Duplicate dependency on package (sexplib0 (and :with-test (>=
v0.15))) in 'depends' field. If you want to specify multiple constraints,
combine them using (and ...).
Hint: To disable this warning, add the following to your dune-project file:
(warnings (duplicate_deps disabled))

We could do as it says (and be forced to bump our dune lang to 3.18). But actually, we can safely bump our own "normal" (i.e. not with-test) dependency of sexplib0 all the way to v0.15 as our lower-bound on ocaml is 4.08.

[NathanReb]

Looks good to me, thanks!