Missed opts when using multiple resumption to model scope-local, reversible variables

Related to #971

[Playground link](https://effekt-lang.org/playground.html?playground=XZDBroMgEEX3fsVd4urlveRtmthF%2F6EfYHVISVAM0lZj%2BPfCADbVDXI4cxmGpKTOQQ2TFvUJF2N0lRktykV2HZUrTJuu1XrdcDP9ekJz5lP4bFU9Scydmeho%2FJTS4qKpAGdXbGEF26IOvx4v5e67nU4%2FHmBpfgyEDdvNx%2FiMASUhepOfUu8%2Bp3N0%2BYKU3rYzD9IzYSee19xKDDzcLpx9UJEzkm2ISCwXxktCn6LmPJ%2BmM7RqDKThxDQqrnm2FkvAv3%2B8nawanR7FkhJDy2X0peko%2F5fNQU%2F9f0Ff%2BTc%3D&repl=bWFpbigp)

The goal of the following code by @marzipankaiser is to (mis)use backtracking & multiple resumption in order to have local operations in a sub-scope that get reverted when we exit the sub-scope:
```scala
effect impl(): Bool
effect exit(): Unit
effect locally{ body: => Unit }: Unit

def scope{ body: => Unit / locally }: Unit =
try {
body()
} with locally {
try {
resume { {b} =>
if (do impl()) {
b()
do exit()
} else ()
}
} with impl {
resume(true)
resume(false)
} with exit { () }
}

def main() = {
scope {
var x = 12
println(x) // ~> 12
do locally {
x = 5
println(x) // ~> 5
}
// now x = 12 again
println(x) // ~> 12
}
}
```