Compute demand signatures assuming idArity

This does four things:

1. Look at `idArity` instead of manifest lambdas to decide whether to use LetUp
2. Compute the strictness signature in LetDown assuming at least `idArity`
   incoming arguments
3. Remove the special case for trivial RHSs, which is subsumed by 2
4. Don't perform the W/W split when doing so would eta expand a binding.
   Otherwise we would eta expand PAPs, causing unnecessary churn in the
   Simplifier.

NoFib Results

--------------------------------------------------------------------------------
        Program         Allocs    Instrs
--------------------------------------------------------------------------------
 fannkuch-redux          +0.3%      0.0%
             gg          -0.0%     -0.1%
       maillist          +0.2%     +0.2%
        minimax           0.0%     +0.8%
         pretty           0.0%     -0.1%
            scc           0.0%     -0.1%
--------------------------------------------------------------------------------
            Min          -0.0%     -0.1%
            Max          +0.3%     +0.8%
 Geometric Mean          +0.0%     +0.0%