Added 'next_idx' attribute to the term table

In order to assure that terms are introduced only with indices greater
that all other indices of term already in the table the term table has
an attribute 'next_idx'. Its value is '1 + last_introduced'.

Furthermore the number of arguments are stored for all variables.

	modified:   ocaml/term_table0.ml
Minor adaption to 'term.ml'

- Fix: 'to_string' was still putting out the argument in reversed order
- 'is_covering' made public

	modified:   ocaml/term.ml