Represent `summonFrom` using a partial function

[nicolasstucki]

Try to represent summonFrom in the compiler as as summonFrom(x => x match { case ... }).

This would mean that we would not have a special kind of Match tree in TASTy to represent it and we could remove reflect.SummonFrom.

[abgruszecki]

During the meeting, I had the impression we aren't going to do this. Do we want to keep this issue around?

[nicolasstucki]

Indeed. I wanted to confirm that tomorrow before closing it.

[nicolasstucki]

We will keep the current representation in TASTy and in the reflection API.