- convert_statement now returns a list of statements which are either
  - converted as a block (for Conditions, loops, etc)
  - flattened when converting a list of statements

The following attributes are registered to Frama-C's list of known
attributes:
- fc_implicit
- fc_pure_template_decl

This is arguably a short-term fix, and a better fix would be to rely
directly on big integers, at least on OCaml side, in the intermediate format.

so that they can be defined in one translation unit while the class is
declared multiple times.

Actually, they appear only in the AST generated by clang-11.

especially if half of them are not seen as implicit afterwards. This can't end
well 

it seems that this is not consistent across clang versions.