iszero for Tangents

[oxinabox]

closes #619
This makes for a nice canonical way to decide if something should be pulled back.
Since even complicated structures like:

julia> iszero([0.0, NoTangent(), [ZeroTangent(), Tangent{Foo}()]])
true

work right.

So this is better than just checking top-level type.
It is much cheaper to check if something is zero, even if it involved elementwise work than the vast majority of pullbacks so i think a case can be made that this should always been done by the AD. Or at least the AD should have special cases for things to treat as as always nonzero like dense numerical mastrixes <:Array{<:Real} (which are also things that rules support).

This should go well with FluxML/Zygote.jl#1389

[codecov]

Codecov Report

Patch coverage: 33.33% and project coverage change: -0.21 ⚠️

Comparison is base (79ba4ef) 94.10% compared to head (cc7b906) 93.90%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #620      +/-   ##
==========================================
- Coverage   94.10%   93.90%   -0.21%     
==========================================
  Files          14       14              
  Lines         899      902       +3     
==========================================
+ Hits          846      847       +1     
- Misses         53       55       +2     

Impacted Files	Coverage Δ
src/tangent_types/tangent.jl	85.25% <33.33%> (-1.02%)	⬇️

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