LazyRow allocations

[touste]

For slightly more complex structs than the one provided in the examples, iterating over LazyRows actually allocates and hurts performance:

using StructArrays, BenchmarkTools

struct Foo
    idx::Int 
    val::Float64
end


sarr = StructArray(Foo(i, 0.0) for i in 1:10000)

val = 1.0

function testalloc(sarr, val)
    for f in LazyRows(sarr)
        f.val = val
    end
end

# This function does not allocate, I just replaced the f.val call by the inlined equivalent.
function testnoalloc(sarr, val)
    for f in LazyRows(sarr)
        getproperty(getfield(f, 1), :val)[getfield(f, 2)] = val
    end
end


@btime testalloc($sarr, $val)
# 220.941 μs (19489 allocations: 304.52 KiB)
@btime testnoalloc($sarr, $val)
# 6.110 μs (0 allocations: 0 bytes)

This is caused by a type instability in the setproperty! function. Any idea how to prevent this?
Thanks

[touste]

Note: this is fixed in PR #153