[gsoc] Remove the `scan` method.

[MariusDrulea]

Motivation and description

Currently the scan method is used to mark the nodes before applying the actual backpropagation in the graph. https://github.com/FluxML/Tracker.jl/blob/master/src/back.jl#L19 We can achieve the same results without employing the this pre-marking of nodes. In this proposal the back function shall not use the scan method, but rather take care of the nodes itself, without a pre-marking. https://github.com/FluxML/Tracker.jl/blob/master/src/back.jl#L74

The functions and files involved are only where the ref field is used: https://github.com/FluxML/Tracker.jl/blob/master/src/Tracker.jl#L41.

The following structures/functions/algos have to be understood in order to remove scan:

1. struct Tracked{T} and it's members
2. the scan, back and back methods in the file back.jl.

Note the function back(g::Grads, x::Tracked, Δ) is used for higher-gradients and we should not focus on this use-case in this proposal.