More work on metadata. Enhanced type, parsing and printing.
May still incomplete work.
Added bitcode writer and nsw/nuw/exact flags to TODO list.
typ_attr -> param_attr (+ added some param keywords).
dc_args: from typ list to (typ * param_attr list) list
Fixed unnamed block and metadata printing.
Removed ll files from master branch. See test branch.
Fixed missing parenthesis around integer for `dereferenceable(n)` attribute.
Fixed return type and parameters attributes in function declaration/definition.
Removed some linkagen kind which are not in LLVM lang ref anymore.
Moved dll storage class in a different type than linkage type.
Completed printer with linkage / dll / visibility / cconv.
Added missing informations to function definition (linkage / cconv / ...)