[InputFile.get_file_contents] is no longer published.

MenhirLib remains under LGPL with a linking exception.
MenhirSdk is considered part of the Generator.
Updated the header of every source file under version control.

Use a new type [input_file] instead, which records the file's name and index
on the command line.
This allows fixing a bug in [UnparameterizedPrinter] where [List.sort] was
used with a non-transitive "ordering".

They need not be exposed outside.

Also, fix the test that $i is within range -- it was incomplete.

  (Now uses the Str module.)

- Lexer.check_producers_indices: New. Are $i keywords correctly used?

- yacc-parser/fancy-parser: Turn the type of actions' semantic values
  into arrows to delay the checking of $i keywords from lexing to
  parsing.

This reverts commit fc203a594b2d68f40b18966550e9c23460fc20e9.

   If the programmer did not provide a name [_i] is used (where [i] is
   the producer's position in the right hand side).

   This syntax modification actually simplifies the code:
   - There is no more on-the-fly naming of unnamed arguments.
   - Inlining is no more restricted to semantic actions that do not use $i.

   The patch is large because:
   - This change has an impact on many modules.
   - The expected results of the bench had to be updated.

Updated the lexer accordingly.
This has the benefit that a type error in a trailer will be correctly located
in the .mly file. Until now, it wasn't.

is used in an OCaml header.