frules/rrules should support receiving Thunks #408
Description
This came up in FluxML/Zygote.jl#966, which wants to support thunks inside Zygote, not just unthunking them as they come out of rrules.
Right now rrules are not explicitly required to support thunks as the input sensitivities. We could enforce this inside ChainRulesTestUtils, and if we do, we should probably decide for 1.0?