Add type arithmetic to improve event type-safety

[nlordell]

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:

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; }'.

[Velenir]

We did the same at contract level in dex-js 😁
I wonder if it makes sense to centralize the types somewhere

[nlordell]

@Velenir I actually stole some of it from your implementation, but I didn't get a discriminated union for the AllEvents type from type AllValues<T extends object> = T[keyof T] But this could be because we have different generated contract types.

[fleupold]

Very cool. Is this something that could be included upstream in the package that creates types for the contracts/web3 ?

[nlordell]

Very cool. Is this something that could be included upstream in the package that creates types for the contracts/web3 ?

Probably! I created #706 so we don't forget.