Implement custom iname tags through a parser registry

Second go at custom iname tags. Iname tags are parsed by the
kernels parse_iname_tag methods, which uses a list of parser
objects from three sources:
* Parsers added to the kernel object through the keyword argument `iname_tag_parsers`.
* Parsers provided by the target through the `iname_tag_parsers` method
* A global parser `default_iname_tag_parser` currently parsing all the
  existent implementation tags.

Parser are expected to return an instance of `loopy.kernel.data.InameTag`
or `None`, if the parser does not accept the given tag. This follows the
idea of how loopy handles function manglers.