Allow more attributes for generic instances

[ALANVF]

This is hopefully a fix for #10550 and any other related issues.

These are the metadata that are now allowed on generic type instances:

• access
• allow
• final
• hack
• internal
• keep (this was already allowed)
• noClosure
• nullSafety
• pure
• struct
• structInit
• using

I'm not entirely sure if using will work as intended, but if it doesn't I guess I could change it.
I'm also not sure what to do with autoBuild, build, and genericBuild so I left them out, and I don't see much use for keepInit and keepSub.

(I probably missed some metadata, so those can be added if needed)

[Simn]

Oh shit, I get to learn new OCaml syntax... :D

And thank you for the contribution!

[ALANVF]

No problem 👍 (also this is by far my favorite ocaml feature, Module.(...)/[/[|/{ opens up Module inside the ...)

[Simn]

So, while I appreciate learning new syntax, I think a shorter and faster solution here would be a good old List.filter.

[ALANVF]

I don't see that that helps here though, and you'd still probably end up using something similar (just a lot longer)