Allow specifying the concrete type of an attribute in format_op

[vaivaswatha]

With 4770b78, the #[format_op] supports specifying attributes in an Op in the syntax, using $attr_name.

This prints (or parses) an AttrObj and gets / sets the corresponding the attribute in the Op. A minor downside of this is that, because we're printing / parsing a general AttrObj, the attribute's attr_id also must be / is printed (so that we know what attribute to parse). A way to avoid this is to allow specifying via a format directive, such as attr($attr_name, "::path::to::concrete::type"). With this, we can generate a printer / parser without the attr_id needing to be part of the syntax.