Unfortunately, even thought all the front-ends use the same
ParserAPI.ml, that file cannot be moved to the folder `shared` due to
a dependency on AST.ml produced by Menhir.
* I added CLI option "--mono" to select the monolithic API of Menhir.
* I added a field "win" to the state of the lexer (a two-token
window for error reporting).
* I escaped LIGO strings before making them OCaml strings (for
example for printing).
