Add most of the dynamic node host implementations

wit/dynamic_node.wit

@@ -11,14 +11,14 @@ interface types {
     resource csv-reader {
         columns: func() -> list<string>;
 
-        next: func() -> csv-row;
+        next: func() -> result<csv-row, string>;
+        next-into-map: func() -> read-map;
         has-next: func() -> bool;
 
         // Get a row by values in one or more columns
         query: func(values: list<tuple<string, string>>) -> csv-row;
 
         read-into-string: func() -> string;
-        read-into-map: func() -> read-map;
     }
 
     resource write-map {
@@ -36,7 +36,7 @@ interface types {
     }
 
     resource read-map {
-        get: func(key: string) -> string;
+        get: func(key: string) -> option<string>;
     }
 }