crate-only mutability

[oli-obk]

in https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Read-only.20fields/near/289150836 @scottmcm asked whether

Could I use keyword generics to be generic over pub(crate) vs mut(crate)?

if we make pub part of the effect logic, we could probably write some sort of where bound, but it seems like an edge case that would only need to exist for supporting read-only fields (writable from the same module)