Previously, we allowed for `Src: Read<A, R>` and `Dst: Read<A, S>` for
different `R` and `S` "reason" parameters. This is an unnecessary degree
of freedom (two types are only ever `Read<A>` for a particular `A` for
the same reason – either because `A = Exclusive` or because both types
implement `Immutable`). This degree of freedom required more verbose
type annotations by callers.

gherrit-pr-id: G3cc35009cf6e17a935df0ae0c3c37dcc389e2f9f