ocaml-sexpt: rename to ocaml-sexp
Drops the redundant 't' suffix that was a typo of 'sexp'. The package
keeps the same OCaml library name (Sexp), API, and tests; only the
on-disk directory name and opam package name change.
Updates:
- merlint/lib/dune, merlint/dune-project, merlint/merlint.opam:
swap the [sexpt] dependency for [sexp].
- merlint/lib/dune.ml: drop the [module Sexp = Sexpt.Sexp] alias
(the new package exports Sexp directly) and switch to the
result-typed [parse_string_many] now that the wrapper module is
gone.
Resolves the build error from earlier in the session where the
merlint/lib/dune still referenced the stale [sexpt] library.