extension function syntax should work on block types too

[kyay10]

In Effekt, you can call a function and provide its first value parameter before the dot:

record bar(a: Int)
def bar2(b: bar) = println(b.a)
def main() = {
  bar(42).bar2()
}

but the same thing doesn't work for block arguments:

effect bar(): Int
def bar2{b: bar} = println(b.bar())
def main() = try {
  b.bar2() // Expected a value, but b is a computation. Use `box b` to pass it as a value instead
} with b: bar { 42 }

(that error message should be improved in the meantime)

[jiribenes]

Related to #1046 which implements this and #845 in which we discussed this further.