Consolidate requests structs under WebDriverRequests

[tristanlabelle]

Changes

• Move all Requests structs under a WebDriverRequests "namespace" (aka enum)
• Remove Session+Requests.swift and Element+Requests.swift, moving the functions to Session.swift and Element.swift
• Remove WebDriverRequest associatedtype ResponseValue for in favor of having only Response, which now defaults to CodableNone. WebDriverRequests.ResponseWithValue<Value> can be specified as the Response type when it follows the typical shape of having a single value property.
• Make requests structs public and add other missing public annotations.

Rationale

This is one further step towards clarifying the layering clear between:

• Low level: WebDriver protocol implementations and request structs. The library can be used exclusively at that layer if needed.
• High level: Session and Element. This layer sits on top of the low-level layer, mostly without exposing it, to provide a more OOP API.

It also makes it easier to see the list of requests structs we have implemented, and makes the Session and Element definitions clearer by being just a list of funcs for each supported operation.

[jeffdav]

This is awesome. This is the sort of thing I'm talking about when I say we should cleanup prior to open sourcing. Don't you feel a lot better about it now? I do!

Also: did you test this against BrowserWin? Just to make sure we didn't break stuff over there?

[tristanlabelle]

@jeffdav Yeah, I have had this API shape in mind for a while and it feels better!

Also: did you test this against BrowserWin? Just to make sure we didn't break stuff over there?

No, but this cannot immediately break BrowserWin thanks to https://github.com/thebrowsercompany/arc/blob/eeeaba1ff34884cecb44c64e0b50f805f69a6c2c/Apps/BrowserWin/Package.resolved#L170 . We might have to do some tweaks next time we update packages. Also, I have a few more improvements in mind ;)