Refactor coercion holes
In fixing Trac #14584 I found that it would be /much/ more convenient if a "hole" in a coercion (much like a unification variable in a type) acutally had a CoVar associated with it rather than just a Unique. Then I can ask what the free variables of a coercion is, and get a set of CoVars including those as-yet-un-filled in holes. Once that is done, it makes no sense to stuff coercion holes inside UnivCo. They were there before so we could know the kind and role of a "hole" coercion, but once there is a CoVar we can get that info from the CoVar. So I removed HoleProv from UnivCoProvenance and added HoleCo to Coercion. In summary: * Add HoleCo to Coercion and remove HoleProv from UnivCoProvanance * Similarly in IfaceCoercion * Make CoercionHole have a CoVar in it, not a Unique * Make tyCoVarsOfCo return the free coercion-hole variables as well as the ordinary free CoVars. Similarly, remember to zonk the CoVar in a CoercionHole We could go further, and remove CoercionHole as a distinct type altogther, just collapsing it into HoleCo. But I have not done that yet.