Note [Default demand on free variables and arguments]

Free variables not mentioned in the environment of a 'DmdType'
are demanded according to the demand type's Divergence:
  * In a Diverges (botDiv) context, that demand is botDmd
    (strict and absent).
  * In all other contexts, the demand is absDmd (lazy and absent).
This is recorded in 'defaultFvDmd'.

Similarly, we can eta-expand demand types to get demands on excess arguments
not accounted for in the type, by consulting 'defaultArgDmd':
  * In a Diverges (botDiv) context, that demand is again botDmd.
  * In a ExnOrDiv (exnDiv) context, that demand is absDmd: We surely diverge
    before evaluating the excess argument, but don't want to eagerly evaluate
    it (cf. Note [Precise exceptions and strictness analysis]).
  * In a Dunno context (topDiv), the demand is topDmd, because
    it's perfectly possible to enter the additional lambda and evaluate it
    in unforeseen ways (so, not absent).