Fix pretty-printing of macro invocations

[catamorphism]

The case in pprust::print_mac for a mac_invoc doesn't print out the body, and there's a FIXME about it. That's about all I can figure out.

[sanxiyn]

Is this still present? I don't see any FIXME in pprust.rs.

[graydon]

It is no longer present. Though the printing is not great. Making it "generally pretty" in all cases is a longer and less-well-defined matter. Closing this for now.