There is no need for wired-in things to go into interface files; the compiler
knows about them anyway.  Worse, it turns ou that if they are in an interface
file, they may get read in with not-quite-right type info (e.g. GHC.Err.error),
and the not-quite-right thing gets into the type envt.  Than it gets used 
instead of the wired in thing.

Best all round never to put them into interface files.  This is the way
it used to be, but it looks as if it rotted away some time ago.

(I noticed this when fixing unsafePerformIO stuff, becuase 'lazy' was getting
an unfolding when it shouldn't.)