Validate the default key name and emit with Xcode runtime warning

[hank121314]

Summary

This PR fixes: #120

1. Implement Xcode runtime warning in Defaults. In order to let Defaults.Key.init support @_alwaysEmitIntoClient, there is a needed to mark some functions/properties as @usableFromInline. And I also explicit the internal keyword for these functions/properties(To remind the developer not to change the access level).

2. Validate the default key name and if it is wrong, emit with Xcode runtime warning.

Screenshot

Thanks

Thanks for your code review 😄 !

[sindresorhus]

In order to let Defaults.Key.init support @_alwaysEmitIntoClient,

Why does Defaults.Key.init need to support @_alwaysEmitIntoClient?

[hank121314]

Why does Defaults.Key.init need to support @_alwaysEmitIntoClient?

With @_alwaysEmitIntoClient, it will let runtime warning appear at call site.

Here is the screenshot if not using @_alwaysEmitIntoClient:

[sindresorhus]

Nice work 👍