feat(eval): add continuation and machine state types

Sources/KiteCore/Kont.swift

@@ -0,0 +1,35 @@
+public struct Winder {
+    public let id: Int
+    public let before: Value
+    public let after: Value
+}
+
+public indirect enum Kont {
+    case done
+    case ifK(then: Value, els: Value?, env: Environment, Kont)
+    case defineK(name: String, env: Environment, Kont)
+    case setK(name: String, env: Environment, Kont)
+    case argsK(pending: Value, accumulated: [Value], env: Environment, Kont)
+    case seqK(remaining: [Value], env: Environment, Kont)
+    case popHandlerK(Kont)
+    case afterHandlerK(handler: Value, continuable: Bool, Kont)
+    case dwBeforeK(before: Value, thunk: Value, after: Value, Kont)
+    case dwThunkK(after: Value, winderId: Int, Kont)
+    case dwAfterK(result: Value, Kont)
+    case rewindK(afters: [Value], befores: [Value], target: [Winder], arg: Value, saved: Kont)
+    case callValuesK(receiver: Value, Kont)
+}
+
+enum MachineState {
+    case evaluating(Value, Environment, Kont)
+    case returning(Value, Kont)
+    case applying(Value, [Value], Kont)
+}
+
+func kontOf(_ state: MachineState) -> Kont {
+    switch state {
+    case .evaluating(_, _, let k): return k
+    case .returning(_, let k): return k
+    case .applying(_, _, let k): return k
+    }
+}