Preserve constraints from

[phadej]

With #185 ghc-tcplugin-api's splitTyConApp_upTo gives us list of coercions used to elaborate applications.

We should thread those through and pass to mkPluginUnivCo & mkPluginUnivEvTerm.

This might be easier to do after #189 if that introduces an own monad for parsing code. Then we could simply add a writer component to it to record encountered coercions, so we don't need to thread them manually.