Two signficant changes to the simplifier

1. Do eta-expansion at let-bindings, not lambdas.
   I have wanted to do this for a long time.
   See Note [Eta-expanding at let bindings] in SimplUtils

2. Simplify the rather subtle way in which InlineRules (the
   template captured by an INLINE pragma) was simplified.
   Now, these templates are always simplified in "gentle"
   mode only, and only INLINE things inline inside them.

   See Note Note [Gentle mode], Note [Inlining in gentle mode]
   and Note [RULEs enabled in SimplGently] in SimplUtils