`some View` for dictionary values

[Jumhyn (Frederick Kellison-Linn)]

Structural opaque result types

EDIT [8/13/21]: Added sections 'Detailed Design - Constraint Inference' and 'Alternatives Considered - Constraint Inference'. EDIT [8/13/21]: Clarified language in 'Detailed Design - Higher Order Functions' and 'Alternatives Considered - Higher Order Functions' thanks to some comments by @ensan-hcl. Introduction An opaque result type may be used as the result type of a function, the type of a variable, or the result type of a subscript. In all cases, the opaque result type must be the entire …