---------------------------------
	Fix a bug in usage recording
	---------------------------------

As a result of the new stuff on hi-boot-file consistency checking, I
accidentally caused Foo.hi to record a usage line for module Foo, and
this in turn caused rather nasty bad things to happen.  In particular,
there were occasional crashes of form

ghc-6.3: panic! (the `impossible' happened, GHC version 6.3.20041017):
        forkM Constructor Var.TcTyVar{d r1B9}

At least I think that's why the crash happened.

Anyway, it was certainly a bug, and this commit fixes it.  The main
payload of this fix is in Desugar.lhs;  the rest is comments and
tidying.