Make define-operator's effect "immediately" visible

* src/debruijn.ml (ectx_get_grammar): New function.

* src/elab.ml (sform_immediate, default_ectx.read_file, lexp_expr_str)
(lexp_decl_str)): Use it.
(lexp_p_decls): Make new scope for body of `let`.

* tests/eval_test.ml (define-operator): No need for workaround any more.

* src/lexer.ml (unescape.split): Use string_sub.

* src/lexp.ml (_lexp_str): Avoid corner case bug.
(get_binary_op_name): Catch corner case bug.

* src/opslexp.ml (clean_map): Prefer `map` when the `i` is not used.

* src/prelexer.ml (prelex_string.getline): Use string_sub.