Adjust inlining heursitics

Adjust inlining heursitics

This patch is the result of a long series of nofib-based experiments
to improve GHC's inlining heuristics.

In the end, I'm not sure how worthwhile it all was: I only got a
   1% decrease in code size
   1% decrease in allocation
and I don't trust the runtime statistics enough to quote.

Still, in doing all this I tidied up the code quite a bit, and
I understand it much better now, so I'm going to commit it.

The main changes are in CoreUnfold, which has lots of new comments.
Other changes:

  - litSize moves from Literal to CoreUnfold
  - interestingArg moves from SimplUtils to CoreUnfold
  - the default unfolding threshold (in StaticFlags)
      reduces from 8 to 6 (since the size calculation
      has changed a bit)