Cleaned up string_of_XXX
Changed the order of arguments for Ind (also started a menhir war)
Changed syntax somewhat, adapted examples to new syntax

* `STAR` is now right associative (as it should be)
* `equiv_terms` have higher precedence than `arrow_terms`
* some terms that used to be simple are not anymore
* parentheses around `ASCRIBE` not necessary anymore
Numerals are valid names, or else `e.1` does not work
Definition of propositional truncation and some basic properties.