moves supercall ensugaring to host

This was one of the few ensugarings when I hesitated wrt their placement.

On the one hand, representing `B(2)` in `class C extends B(2)` as an Apply
is syntactically natural, even if a bit awkward.

On the other hand, applications of types to terms are particularly incompatible
with scalac's infrastructure (e.g. app.symbol is a type symbol, not a superctor symbol),
and that bit me when I implemented name/default desugaring for supercalls.
Therefore, I'm reverting the original decision to have supercall processed
in ensugar and moving it to host.
moves the list of supported ensugarings to the top of the file