accounts/abi: fixed unpacking in to already slice interfaces

Previously it was assumed that wheneven type `[]interface{}` was given
that the interface was empty. The abigen rightfully assumed that
interface slices which already have pre-allocated variable sets to be
assigned.

This PR fixes that by checking that the given `[]interface{}` is larger
than zero and assigns each value using the generic `set` function (this
function has also been moved to abi/reflect.go) and checks whether the
assignment was possible.

The generic assignment function `set` now also deals with pointers
(useful for interface slice mentioned above) by dereferencing the
pointer until it finds a setable type.