Approach Resource typing differently in the UI (#58)

Previously the Resource typing was awkward at best - asserting Resources
to concrete types seamed like needles work that defied the point of
having them defined as interfaces

One potential solution was asserting string literal to string literal,
but that felt backwards, example: "literal" as "literal"

Someone in the Github thread[0] suggested asserting them in the
following manner: "literal" as const - which is a bit better, but still
misses the point

Due to technical reasons the literal inferred type was string, rather
than the literal itself. What seams to solve the problem is asserting
the whole object to be a Resource. That also solves the problem with
creeping import scope while at the same time setting up correct guards
at compile time

Last, but not least I think that the documentation on type assertion is
misleading at best (couldn't find an example describing the mechanism
and didn't feel like going through the source) - the mechanism is much
more safe than described in the official docs, which cover the case of
any -> T. Whereas in the more general case of T -> U assertion the
overlap is checked (so that if I make a typo in the object an error is
raised)

[0] microsoft/TypeScript#25889

src/App.tsx

@@ -27,8 +27,8 @@ export default class App extends Component<{
     {
         super(props);
 
-        // TODO: get rid of the casting
-        const ourTree = makeTree({__typename: "Text", content: "korzen", is_done: false} as Text);
+        const initial_resource: Resource = {__typename: "Text", content: "korzen", is_done: false};
+        const ourTree = makeTree(initial_resource);
 
         this.state= {
             chosenNode: ourTree.root.id,
@@ -206,11 +206,10 @@ function displayTree(t: Tree<Resource>, chosenNode: NodeID): ReactTreeGraphNode
 }
 
 
-// TODO: get rid of the casting
 function displayResource(r: Resource): string
 {
     return {
-        "Link": (r as unknown as Link).address,
+        "Link": (r as Link).address,
         "Text": (r as Text).content,
     }[r.__typename];
 }

src/ResourceAdder.tsx

@@ -1,5 +1,4 @@
-// TODO: how to limit importing scope?
-import { Resource, ResourceTypeString, ResourceTypeStringValues, Link, Text } from "./core/resources";
+import { Resource, ResourceTypeString, ResourceTypeStringValues } from "./core/resources";
 
 import React, { FunctionComponent } from "react";
 import { Formik, Form, Field } from "formik";
@@ -40,11 +39,10 @@ const ResourceAdder: React.FunctionComponent<{
 export default ResourceAdder;
 
 
-// TODO: get rid of the casting
 function default_for_typestring(t: ResourceTypeString): Resource
 {
     return {
-        "Link": { __typename: "Link", address: "", is_done: false } as Link,
-        "Text": { __typename: "Text", content: "", is_done: false } as Text,
+        "Link": { __typename: "Link", address: "", is_done: false } as Resource,
+        "Text": { __typename: "Text", content: "", is_done: false } as Resource,
     }[t];
 }

src/ResourceEditor.tsx

@@ -27,7 +27,7 @@ const ResourceEditor: React.FunctionComponent<{
                 __typename: props.resource.__typename,
             };
 
-            props.onEditonCommit(resource as any);
+            props.onEditonCommit(resource as Resource);
         }}
     >
         {