Rename parser to namespace

In the first implementation, the namespace was stored inside the parser object, but we realized that this information was needed in many more places than in the parser (I/O, restart, messages, etc).

Although we could keep it this way and use the parser to get the namespace where needed, this would look awkward. Instead, the simplest thing to do is to rename the parser_t data type, that only contains the name space, to namespace_t and move it to its own module.