Specialize fill!

[jishnub]

This helps considerably with sparse parent arrays by dispatching to specialized methods instead of hitting a fallback.
On master

julia> using StructArrays, BandedMatrices, BenchmarkTools

julia> A = BandedMatrix(0 => rand(100));

julia> B = BandedMatrix(0 => rand(100));

julia> S = StructArray{Complex{eltype(A)}}((A,B));

julia> @btime $S .= 0;
  66.965 μs (0 allocations: 0 bytes)

This PR

julia> @btime $S .= 0;
  19.845 ns (0 allocations: 0 bytes)

[jishnub]

@aplavin Could you have a look at this PR?

[aplavin]

I think it's better to follow the insert! function above

StructArrays.jl/src/structarray.jl

Lines 403 to 407 in 8c7c75a

	function Base.insert!(s::StructVector{T}, i::Integer, vals) where T
	valsT = maybe_convert_elt(T, vals)
	foreachfield((v, val) -> insert!(v, i, val), s, valsT)
	return s
	end

and use maybe_convert_elt.

Btw I can only comment here, not actually confirm merging.

[piever]

Thanks for this! Other than for some small stylistic comments it looks good to me.

[jishnub]

I've included the changes, could you have a look again?

[piever]

LGTM!