Fix merlint issues: E335, E605, E718 in ocaml-sqlite; fix gen_corpus
- Remove underscore prefix from used binding in lexer.ml (E335)
- Add test_ast.ml and test_lexer.ml for schema parsing coverage (E605)
- Replace hand-written gen_corpus.ml with alcobar --gen-corpus rule
in ocaml-sqlite, ocaml-scitt, and ocaml-cdm fuzz dirs (E718)