[decoder] Исправлены тайпинги

decoder/index.ts

@@ -201,7 +201,7 @@ export class Pure<A> extends DecoderBase<A> {
 export class Chain<A> extends DecoderBase<A> {
   constructor(
     readonly _decoder: Decoder<any>,
-    readonly _then: (x: Validation<any>) => Decoder<A>,
+    readonly _then: (x: Either<Problem, any>) => Decoder<A>,
   ) { super(); }
 }