RFC: gencap-write

[Theodus]

Add #write, a new gencap for use in type parameter constraints, implying the capability set: {iso, trn, ref}.

https://github.com/ponylang/rfcs/blob/master/text/0043-gencap-write.md

[SeanTAllen]

@jemc it looks like you had this almost finished when the rfc went in, do you want to finish this off?

[jemc]

Sorry about this.

It turned out that this gencap didn't actually solve my problem (5 years ago), and neither I nor anyone else has had a pressing reason to add this.

Additionally, now (5 years later) that I know more about the subtle edge cases with generic subtyping, I'm a lot more wary about adding new generic caps, since that means more tricky edge cases.

So if nobody is actively needing this feature, and it risks introducing new bugs, I suggest we don't continue with this at this time.