* src/eval.ml (from_lctx): Rewrite to obey the db_offset

* src/debruijn.ml (lexp_ctx_cons): Tighten the check.

* src/env.ml (value_type): Use runtime_env.
(runtime_env): Drop the extra two integers.
(env_cell): Make the var's name immutable.
(get_rte_variable, get_rte_size, add_rte_variable, print_rte_ctx):
Adjust accordingly.
(set_rte_variable): Adjust and make return type be unit.
(get_rte_size, is_free_var, rte_shift): Remove.

* src/myers.ml (fold_left, fold_right, map, iteri): New functions.

* tests/env_test.ml: Don't use the return value of set_rte_variable.