Make StructArrays broadcast aware

[Keno]

Fixes #89

cc @mbauman who can say whether this is right

[codecov-io]

Codecov Report

Merging #90 into master will decrease coverage by 0.22%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
- Coverage   91.08%   90.86%   -0.23%     
==========================================
  Files           9        9              
  Lines         370      372       +2     
==========================================
+ Hits          337      338       +1     
- Misses         33       34       +1

Impacted Files	Coverage Δ
src/structarray.jl	85.85% <50%> (-0.74%)	⬇️

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[piever]

Thanks! I'll need @mbauman review as well as I'm not super familiar with the broadcast machinery.

The example

s = StructArray{ComplexF64}((rand(2,2), rand(2,2)))
s .+ s

is very convincing, but what should one expect from

s = StructArray{ComplexF64}((rand(2,2), rand(2,2)))
t = rand(2,2)
s .+ t

Is the rule that, in order for the result to be a StructArray, all members of the broadcasted operation have to be StructArrays and the resulting eltype a struct?

[piever]

Probably this also needs to test that things like broadcast(t -> 1, s) don't give an error (I'm not sure what exactly should happen when the result of broadcasting StructArrays has a eltype that is not a struct).

[vchuravy]

This won't work for the case where your StructArray contains a different array than Base.Array. Normally you have to unwrap the wrapper and do the broadcast on the underlying array to reach the right implementation.

[cossio]

Could this help with #124?

[piever]

Superseded by #136.