Cast API overhaul (#173)

Re-write the shim<->bridge messaging interface and session creation/update handling for better accuracy.

app/src/global.d.ts

@@ -1 +1,6 @@
 declare type Nullable<T> = T | null;
+
+declare type DistributiveOmit<T, K extends keyof any> =
+        T extends any
+            ? Omit<T, K>
+            : never;