Add RFC for none and unknown types

rfcs/none-and-unknown-types.md

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+# Add none and unknown types
+
+## Summary
+
+Add top and bottom types that ane inhabited by everything and nothing respectively.
+
+## Motivation
+
+There are lots of cases in local type inference, semantic subtyping,
+and type normalization, where it would be useful to have top and
+bottom types. Currently, `any` is filling that role, but it has
+special "switch off the type system" superpowers.
+
+## Design
+
+Add:
+
+* a type `none`, inhabited by nothing, and
+* a type `unknown`, inhabited everything.
+
+Use them, rather than `any` where appropriate. Ideally, we would then never infer `any`.
+
+## Drawbacks
+
+Another bit of complexity budget spent.
+
+These types will be visible to creators, so yay bikeshedding!
+
+## Alternatives
+
+Stick with the current use of `any` for these cases.