an important fixup for macro universe API
an important fixup for macro universe API

It seems that this.type doesn't cut it for stuff declared in a trait.
Downgrading this.type to Symbol and Type correspondingly in
SymbolContextApi and TreeContextApi.

felixmulder/scala,felixmulder/scala,slothspot/scala,lrytz/scala,jvican/scala,scala/scala,lrytz/scala,lrytz/scala,slothspot/scala,shimib/scala,slothspot/scala,lrytz/scala,scala/scala,jvican/scala,martijnhoekstra/scala,slothspot/scala,jvican/scala,scala/scala,martijnhoekstra/scala,scala/scala,lrytz/scala,shimib/scala,jvican/scala,jvican/scala,martijnhoekstra/scala,felixmulder/scala,scala/scala,jvican/scala,felixmulder/scala,felixmulder/scala,felixmulder/scala,slothspot/scala,shimib/scala,jvican/scala,shimib/scala,martijnhoekstra/scala,shimib/scala,shimib/scala,felixmulder/scala,slothspot/scala,scala/scala,martijnhoekstra/scala,martijnhoekstra/scala,slothspot/scala,lrytz/scala