Arrayable is unusable

[fregante]

Initially reported in #935 (comment)

In short:

• function castArray<T>(a: Arrayable<T>): T[] is hard to type
  Demo
• The code example in the type's source produces any[], losing the type information provided
  Demo
  

cc @Max10240 let's continue here

Upvote & Fund

• We're using Polar.sh so you can upvote and help fund this issue.
• The funding will be given to active contributors.
• Thank you in advance for helping prioritize & fund our backlog.