Adjoint not defined on AbstractThunk #224
Description
It would be useful to define adjoint on thunks.
Some rules call adjoint internally, .e.g. Cholesky,
and defining adjoint would make it possible to call them without having to unthunk first.
adjoint is a linear operator so it should be fine to define.
Would be useful for invenia/Nabla.jl#189
(came up in the tests for Cholesky)