Add a record with 2 fields : tnode and tattr

For now we ignore "unused" label attribute, because the current mecanism
does not support different attributes with the same name : "unused" can
be a label attribute or a function attribute.

also add `@since` and `@before` odoc tags

- now, its a single constructor
- also adds a boolean logic constant

- Always print plugin except kernel
- The only optional plugin is used in extension_from and plugin_from
- All other acsl_extension functions take a plugin:string
  - these functions raise Not_found is the extension is not registered
  - can also fatal in case of ambiguity if plugin is None

- Plugins fields in the AST are used to find the right extension
- A plugin still cannot register twice an extension
- If plugin is not given, can throw ambiguity error
- Kernel extensions are an exception, a plugin cannot register an
extension with the name name than the kernel

- Cil_types.acsl_extension
- Logic_ptree.extension & global_extension