Fix pretty printing of parentheses in types

Some types generate superfluous parentheses:

 #print_type ('a | 'b ) \'c;;
(('a | 'b) \ ('c))

And some others are missing parenthses:

#print_type (Int -> Int) & (Bool -> Bool);;
Bool -> Bool & Int -> Int
# #print_type Bool -> Bool & Int -> Int;;
Bool -> Arrow

here we cannot paste the output of the toplevel.

Edited May 02, 2021 by Kim Nguyễn