🤞🏼 Rephrase async code as PromiseKit Promises

[chris-araman]

RE: #346 (comment)

Using Combine would seem to be the future-proof option, as that's the API Apple has produced for asynchronous programming. However, we can't use Apple's Combine yet because it requires macOS 10.15. There are some open source implementations that support older macOS releases.
🔴 CombineX failed swift build.
🟡 I made some progress with OpenCombine, but got stuck when I found that it doesn't yet implement merge.

So, here's the async code rephrased as PromiseKit promises.

👍🏼 Pros:

• 89 fewer LOC in mas and MasKit.
• Arguably improved readability.
• PromiseKit is well documented and seems to have good test coverage.

👎🏼 Cons:

• The default PromiseKit configuration assumes a UI app. MasKit.initialize configures PromiseKit for use in a console app, where we regularly wait on the main queue for promises to complete on the global queue. main.swift and all test case entry points invoke MasKit.initialize.
• It's another dependency to maintain.
• The mas executable (arm64, release) increases in size by ~388 KiB, or ~53%. I might be able to reduce this with further investigation, but considering we just shed ~13 MiB from MasKit.framework, I think it's probably fine.

I'm open to keeping this or dropping it. I'll leave it up to your best judgment, @phatblat.

ℹ️ Commit fb01f5b is here only to maintain existing behavior. With this commit, commands that download (or purchase) multiple apps will continue to attempt all downloads even if an earlier download fails. Without this commit, we'll stop downloading at the first failure. I'm inclined to think the behavior without this commit (failing fast) is what users would expect, but perhaps it's common for one download to fail while others succeed for reasons unrelated to network failures. If you agree, feel free to pop that commit off the stack, or ask me to do so, before merging.

[phatblat]

I want to get the release out before we make this change. Also want to give time to review it properly.

[chris-araman]

Fixes #344. With the async logic now described in terms of promises, implementing retry logic was pretty straightforward.

caraman@mac ~/s/mas (promisekit)> swift run mas install 462062816
==> Downloading Microsoft PowerPoint
Warning: The Internet connection appears to be offline.
Trying again up to 2 more times.
==> Downloading Microsoft PowerPoint
Warning: The Internet connection appears to be offline.
Trying again up to 1 more time.
==> Downloading Microsoft PowerPoint
###################################################--------- 85.0% Installing

[phatblat]

One idea for the tests