* unify various checks for whether a suspend function needs a
continuation;
* mark the continuation parameter with an origin (this also allows
correctly computing the offset of the local in codegen -- see the
new test);
* when wrapping `suspend fun main`, use a suspend reference instead of
a synthetic non-suspend lambda (required to correctly implement the
previous point, as previously the continuation parameter was passed
to main() itself correctly only because of very lenient checks in
AddContinuationLowering).
pyos