Minor Changes, most notably singleton types

[bobbybob2131]

Singleton types were recently added (see https://devforum.roblox.com/t/luau-recap-march-2022/1738645 and https://github.com/Roblox/luau/blob/master/rfcs/syntax-singleton-types.md), this PR just updates Fusion to utilise them. All edits are based on existing comments. I also removed a completed (I think?) todo note.

Edit: Sorry for the close/reopen, I thought I needed to make a new PR to add the second commit but Github does it automatically. Edit: Remove table.pack note

[dphfox]

We use table.pack because it gives us a n field, which can later be used with table.unpack to ensure we don't miss out values returned after nils.

[dphfox]

Closing in favour of a newer PR #215