Add constrained types RFC

[SeanTAllen]

No description provided.

[rhagenson]

Formatting edits that I saw. There is one incomplete sentence as well.

I like the idea of including this in either stdlib or ponylang organization.

[SeanTAllen]

There was a comment from @adrianboyko that was on a commit and doesn't appear in the UI. It was a good comment about the naming of ValidationErrors. @adrianboyko can you repeat the comment for everyone.

At any rate, I made the suggested change.

[adrianboyko]

My suggestion was this: Since the opposite of "success" is "failure", the counterpart of ValidationSuccess should be ValidationFailure and not ValidationErrors. The errors are an attribute of the failure, accessible through fun errors(). The errors explain the failure but they are not the failure itself.

[SeanTAllen]

Accepted.