Two things here:

* While debugging Trac #14561 I found it hard to understand
  ghcPrimIds and magicIds in MkId.  This patch adds more
  structure and comments.

* I also discovered that ($) no longer needs to be a wiredInId
  because we now have levity polymorphism.  So I took dollarId
  out of MkId; and gave it a levity-polymorphic type in GHC.Base