I removed the last top-level effect (the execution of cpp).
The idea is that ParserUnit.ml and each ParserMain.ml get closer
to pascaligo.ml, cameligo.ml and reasonligo.ml, respectively.
* Duplicate variants in the same type declaration
* Duplicate parameter in the same function declaration
* Shadowing of predefined value in a declaration
I fixed the architecture for that support: ParserMain.ml is now where
those specific errors are handled, and they are produced by the
semantic actions of the parsers.
* 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).
I also had to remove the keywords [Down], [Fail] and [Step] in
PascaLIGO that made a mysterious and unwanted come back. (I did not
bother with [git blame]).
