Transfer strictness and arity info when abstracting over type variables

Transfer strictness and arity info when abstracting over type variables

See Note [transferPolyIdInfo] in Id.lhs, and test
	eyeball/demand-on-polymorphic-floatouts.hs

Max Bolingbroke discovered that we were gratuitiously losing strictness
info.  This simple patch fixes it.  But see the above note for things
that are still discarded: worker info and rules.