ReadonlyDeep: Fix handling of objects with call signatures

[RebeccaStevens]

partial fix of #337

Makes objects with a call signatures readonly.

Limitations:

• An object with both properties and multiple call signatures cannot be mapped to another type without losing the call signatures.
  • see Overload gets lost in mapped type with conditional type microsoft/TypeScript#29732

Current this PR assumes that any object that has an call signatures is not a map or set. I can extends this PR to handle maps and sets with call signatures.

[sindresorhus]

Current this PR assumes that any object that has an call signatures is not a map or set. I can extends this PR to handle maps and sets with call signatures.

Do you think it should be handled?

[RebeccaStevens]

@sindresorhus It's a rare case but probably should be handled for completeness.

Edit: Actually I don't think it's possible to to make a native map or set callable. Thus this will be a very rare case.

[sindresorhus]

It's a rare case but probably should be handled for completeness.

I agree. It would be great if you could handle it then.

[RebeccaStevens]

@sindresorhus I've made that change locally to support maps and sets but then I realized that with how this type currently works, only native maps and sets are supported.
That is to say if we have the type Set<string> & { customProp: string } the ReadonlyDeep version of this would just be the same as ReadonlyDeep<Set<string>>.

As native map and set can't be made callable (as far as I know), my local change will have no benefit.
If we want to support non-native maps and sets, that should probably be done in it's own PR.

I'd say the PR is currently ready in its current state.

[sindresorhus]

Thanks :)