Update of the README/INSTALL files to remove mention of .opt binaries and give updated info about opam

Fixes bad generation of variable names (including non-ascii characters) in datalog programs when there are more than 26 of them

change the alias name for checking whether all the geretared error messages have a user friendly wording from 'check' to 'check_all_messages' after dune added a '@check' alias (1.5.0)

Few fixes in dune files. Prepare an opam branch that will be compatible both with menhir >= 20181006 and earlier versions.

nl_lexicon now restricts the non linear interpretation to the interpretation of -> and lambda at the abstract level. It is the user's responsability to have Lambda's and => in the lexicon and in the object signature when required. Fixes #12

Restrict the effects of nl_lexicon to de-linearization of abstract -> and abstract lambda only. Fixes #12