Add a new flag print_types_of_subterms

[aqjune]

This patch adds a new int ref flag print_types_of_subterms.
When it is 0, pp_print_term does not print the types of subterms.
When it is 1, as it is by default, pp_print_term only prints types of subterms containing invented type variables.
When it is 2, pp_print_term prints the types of all constants and variables in the term.

This also changes the behavior of print_term to print the invented type variables inside a term as well.

  # loadt "Library/words.ml";;
  ...

  # `word_join (word 10:int64) (word 20:int64)`;;
  Warning: inventing type variables
  val it : term =
    `(word_join:(64)word->(64)word->(?194629)word) (word 10) (word 20)`
  # `word_join (word 10:int64) (word 20:int64):int128`;;
  val it : term = `word_join (word 10) (word 20)`

  # print_types_of_subterms := 0;;
  val it : unit = ()
  # `word_join (word 10:int64) (word 20:int64)`;;
  Warning: inventing type variables
  val it : term = `word_join (word 10) (word 20)`
  # `word_join (word 10:int64) (word 20:int64):int128`;;
  val it : term = `word_join (word 10) (word 20)`

  # print_types_of_subterms := 2;;
  val it : unit = ()
  # `word_join (word 10:int64) (word 20:int64)`;;
  Warning: inventing type variables
  val it : term =
    `(word_join:(64)word->(64)word->(?194609)word) ((word:num->(64)word) 10)
    ((word:num->(64)word) 20)`
  # `word_join (word 10:int64) (word 20:int64):int128`;;
  val it : term =
    `(word_join:(64)word->(64)word->(128)word) ((word:num->(64)word) 10)
    ((word:num->(64)word) 20)`

[aqjune]

Addressed comments. :)