feat: improved types

[aleclarson]

The current types were broken in TS 3.6+ so I partially rewrote them.

And fromDepth was misspelled depthFrom.

[coveralls]

Coverage remained the same at 100.0% when pulling 9963c59 on aleclarson:patch-1 into 04b45e7 on blakeembrey:master.

[blakeembrey]

What was the reasoning behind this switch between tuple and array behavior? I wasn't able to replicate the reason this was here when testing different solutions.

[blakeembrey]

3.7 included recursive types, but it didn't improve for within conditions - I was still getting the same error when testing this out. It's the main reason I still need to do:

{
  [K in Extract<keyof T, number>]: PickValue<T[K]>;
}[number]

As long as it wasn't any sort of explicit tuple support, that's fine - I made sure I have tests for tuples on master but wasn't sure if this did something I failed to test.

[aleclarson]

Did you intentionally exclude the arguments support?

T extends IArguments
  ? unknown
  : // ...

[blakeembrey]

@aleclarson Not intentionally, it wasn't clear that line was there to just make it be the unknown type instead of any. I'm not sure how useful it is to make it unknown now that I think about it, but there's definitely a test for arguments on master that passes.

[aleclarson]

unknown forces you to use type guards or an explicit typecast, so less runtime errors

[blakeembrey]

I’m aware of how it works. To clarify, I don’t see the point of changing a type on someone in this library if they’re using something unsafe - otherwise we could change all any types. I’d hope arguments usage is an edge case in TS, and by putting it in this library it’s condoning the usage arguments as the input type - which I believe it shouldn’t be used as for performance reasons.

[blakeembrey]

I believe I was able to simplify this a bit (after my first attempt using 3.7 recursive type features failed on ReadonlyArray:

export type PickValue<T> = T extends ReadonlyArray<any>
  ? {
      [K in Extract<keyof T, number>]: PickValue<T[K]>;
    }[number]
  : T;

export type FlatArray<T extends ArrayLike<any>> = Array<PickValue<T[number]>>;