WIP: Make getindex work for arrays with any eltype

[oxinabox]

This is split off ove #240

@willtebbutt said here #240 (comment)

This broadly LGTM. It would be good to add some tests involving arrays with an element type that's not a number -- perhaps an array of arrays?

Turns out this is much harder than i thought.
Finding zeros is annoying
Splitting it into its own PR so not to block other more important things

I think this is right but it could be better and also:
ChainRulesTestUtils freaks out at these tests.

I know one bit to fix it is to do LinearAlgebra.norm(::Zero) = 0.0
I think another might be to stop using isapprox directly on arbitary types

One thing that makes it a bit more annoying is:
JuliaLang/julia#38064