Add type arithmetic to improve event type-safety (#705)

This PR just adds some utility types that improve type-safety on event types. They are currently not used, but I figured that it was worth including it in a separate PR since it is rather dense code that takes some time to get used to.

### Test Plan

Example of it in action:
```ts
const eventData: AnyEvent<BatchExchange> = {} as any
switch (eventData.event) {
case "Token":                              // ERR
  break
case "OrderPlacement":
  eventData.returnValues.buyToken = "asdf" // OK
  break
case "Withdraw":
  eventData.returnValues.buyToken = "asdf" // ERR
  break
}
```

Produces the folling `tsc` errors:
```
src/streamed/events.ts(50,8): error TS2678: Type '"Token"' is not comparable to type '"OrderPlacement" | "TokenListing" | "OrderCancellation" | "OrderDeletion" | "Trade" | "TradeReversion" |
 "SolutionSubmission" | "Deposit" | "WithdrawRequest" | "Withdraw"'.
src/streamed/events.ts(56,28): error TS2339: Property 'buyToken' does not exist on type '{ user: string; token: string; amount: string; 0: string; 1: string; 2: string; }'.
```

src/streamed/events.ts

@@ -0,0 +1,43 @@
+import { Contract, EventData } from "web3-eth-contract"
+import { ContractEvent } from "../../build/types/types"
+
+/**
+ * Event type specified by name.
+ */
+type Event<
+  C extends Contract,
+  T extends Exclude<keyof C["events"], "allEvents">,
+> = EventMetadata & EventDiscriminant<C, T>
+
+/**
+ * Concrete event type with known properties based on the event name.
+ *
+ * @remarks
+ * This type definition allows the TypeScript compiler to determine the type of
+ * the `returnValues` property based on `event` property checks. For example:
+ * ```
+ * const eventData: AnyEvent<BatchExchange> = ...
+ * case "Token":                              // ERR: 2678: Type '"Token"' is not comparable to type
+ *                                            // '"OrderPlacement" | "TokenListing" | ...'
+ *   break
+ * case "OrderPlacement":
+ *   eventData.returnValues.buyToken = "asdf" // OK
+ *   break
+ * case "Withdraw":
+ *   eventData.returnValues.buyToken = "asdf" // ERR: 2339: Property 'buyToken' does not exist on type
+ *                                            // '{ user: string; token: string; amount: string; ... }'.
+ *   break
+ * }
+ * ```
+ */
+type AnyEvent<C extends Contract> = EventMetadata & EventDiscriminant<C, Exclude<keyof C["events"], "allEvents">>
+
+type EventMetadata = Omit<EventData, "event" | "returnValues">
+type EventValues<T> = T extends ContractEvent<infer U> ? U : never
+type EventDiscriminant<
+  C extends Contract,
+  T extends Exclude<keyof C["events"], "allEvents">,
+> = T extends any ? {
+  event: T,
+  returnValues: EventValues<C["events"][T]>,
+} : never