* src/env.ml (value_type) <Vtype>: New, replaces Vdummy and Vinductive

(value_equal): Adjust accordingly.  Emit a warning for Vundefined.

* src/eval.ml (make_symbol, make_string, make_integer): Simplify.
(make_float): Implement.
(make_block): Comment out.
(read_impl, write_impl): Simplify.
(eval_call): Adjust Vinductive case to the new Vtype and tweak it so
that the result can itself be passed to eval_call, in case the
inductive type is applied to more than one argument.
(_eval_decls, y_operator): Use Vundefined instead of Vdummy.
(register_built_functions): Comment out `block_`.

* src/lexp.ml (_lexp_str): Remove unreachable case.
(lexp_str.septp): Remove, unused.

* btl/builtins.typer (block_): Comment out, not implemented.

* src/elexp.ml (elexp): Unify Type and Inductive.  Keep lexp in `Type`.

* tests/env_test.ml (_.rctx): Use Vundefined instead of Vdummy.